Please only post if you are fairly informed, the other posts I found were just a bunch of whining about it being a massive buff cause slow on W and E changes with new passive and ignored the ratio Q halving along with W damage. Just looking for some genuine insight on my favorite champion :D
Flat out nerf:
weaker laningphase, weaker midgame and even in the lategame she lost a lot of dmg in most circumstances, as you rarely can just go in and spam your e in fights, most of the time you´ll deal the bigger part of your dmg in teamfights from range with your dots and only go in for the e rto clean up a fight.
I only have a couple of games in now, but some things that really jump at me are:
Once you start to get the heavy-hitting AP items going, things will start to look up (hopefully you didn't get trounced in the early game. Oh, and if you build any CDR, you're wasting your money, so all those items (Chalise for example) are unusable). Late-late game I've yet to reach, but the damage started to look good, so long as you get to actually deal it (short range is short), but again, Q and W deal little damage and you need that poison on the target.
EDIT: Things that are good: E reliability seemed good and the new R point blank AoE is a godsent.
Overall? I feel this was an unintentional nerf, or I'm just building (and playing) the character horribly wrong, though I have decent experience from before the change.
I thinks it's a plain nerf, her earlygame was gutted quite hard, her manacosts are insane without the old passive. She might toss anivia from the throne off the most manahungry champs now...
All she gains is a massive lategame she probably never reaches....
Imo, her Q should recive some higher ration, maybe not the old one, but atleast 0.5, and her E should refound atleast 50%, rather more, of the manacosts upon reset. To balance that out, simply reduce the %ap from her passive to 20%, or even 15%
Its a damage shift - so whether you consider it a nerf or buff is up to you - but her rank 5 twin fangs is an obvious damage buff - her 3 utility spells got nerfed damage wise, but received utility buffs to them--- overall less early game power and stronger late game power with the late game buffs of damage and utility - and her really strong literally non-existent passive early game and the best passive late game
i dont really think an extra 30percecnt is worth the q ratio droped i mean i usally end up with 500 ish ap with her so 30 percent of that is 650 which i guess is better for her main damange twin fang but with her q ratio and her ult damange gone i dont think its worth unless your super far ahead
I have no idea why everyone is QQing and saying nerf. I have had zero problems with new Cass, and I like her gameplay. She's a lot more difficult to use since she's now Nasus-esque and requires stacks, but there's some good stuff in her kit. It's the same whines I hear with Jayce, despite the fact they both have passive abilities that restores their mana outside of their regeneration. All you need on Jayce is triforce and if you're good enough with him, your mana problems will go away. All you need on Cass is Rod of Ages, and if you're good enough with her, your mana problems will go away.
increasing the cd on Q and adding to the delay has made her unplayable new skin model makes her look like she has feathers and its plain and boring looking with all detail removed
She doesn't do any damage anymore. You have to build mana to keep her from being oom early because of her spamming, even with the e passive, and her bases are so low that she feels useless. Your q, which used to do insane amounts of damage, does like 45 a tick, even late game on an adc, her w is basically useless except for zoning, and her ult is just utility with very little damage behind it (terrible utility at that, what with people being able to turn around as it is casting). You would think that would require huge buffs to her e's damage, but no, they give it a slight buff in both damage and scaling, and by slight I mean 35 damage and .05% scaling, and that's only when maxed, early it has huge nerfs. I love the new skills but she feels like a rainy day, annoying but easily bearable.
Cassiopeia is in dire need of some buffs/numberchanges, there's no denying it. I've said it plenty of times already but this rework is a downright nerf.
I've played over 20 games with the new Cass so far and I still haven't managed to surpass the damage-output of the old one, no matter how often I looked at the damage-charts on the result screen. Infact our ap supports constantly outranked me in total damage dealt to champions. So much for wanting her to deal "tons of damage".
They probably never realized how impractical this utter reliance on her E is. Combined with her horrendous mana costs there's literally no reason to use it for anything but to farm, doing otherwise only hurts your sustain in lane.
There's a good reason why the old Cass resorted to her Twin Fang as a mean to finish off weakened opponents. The game is mainly played via poke and harass (all things which were TAKEN from her), not by going ham right off the bat. When will they realize that? There are assassins for this role and Cass lacks the burst and mobility to be one.
Her overall performance is far too delayed and entirely unreliable since there's no guarantee you'll ever reach lategame. This rework has turned Cassiopeia into a liability, not a hypercarry.
a complete gutting Only improvement was the hit detection on poison was finally fixed.
Ratios removed. R is still slow as hell to use. http://forums.na.leagueoflegends.com/board/showthread.php?p=49873177#post49873177
Terrible.. Riots reworks.. idk what to say.
also max Q first as usual. Still leaves you far less mana hungry in lane phase. you can still use W to clear ranged creeps/whole wave if you want. But it does no damage like Q because they both got nerfed to shit on their ratios.. So double dorans cass is pointless. might as well starts dorans and rush the tear/roa/rylias path and get liandries at some point. Constantly harassing to get the passive to 400 before the 30~ minute mark
its a huge buff! 30% extra AP for free without even picking up a Deathcap +25% CDR and healing per E its massive!
late game with 
she does around 790 dmg an E and heals around 120 pts at 0.5 seconds an E on a poisoned target (yeah thats one shotting minions late game and melting midlaners like they are butter)
Both really, she is the mage equivalent of Kog'Maw now. Somewhat weak early, but once she gets to a certain point her damage is godlike, yet even then still has huge weaknesses that can be exploited.
Her lack of zoning, crowd control and tankiness/mobility leaves her exposed (especially to assassins / divers), so if she's picked the team comp has to be built around her just as with Kog'Maw. Expect to do a ton of farming for 30 minutes until you acquire 3-4 items and at least 200 stacks on your passive (by then you should be near maxed though). Engaging before that point will leave you at at a disadvantage.
Nerf. Cassiopeia portrait now looks like Wukong.
It took me a long time to get a chance to play her yesterday, but when I did, the game I got into ended up being a 4v5 with the enemy mid laner missing. So as new Cassiopeia, this should have been the ideal scenario: Free lane I can use to scale hard into the late game. Turns out that instead I discovered a major flaw in Cassiopeia's new passive.
It turns out that new Cassiopeia has an insanely difficult time getting stacks of her passive without an enemy champion to stack her passive with.
Think of it this way: With no enemy champion in lane to deal with, Cassiopeia has to rely on last hitting poisoned minions in order to get her stacks. So I need to last hit 400 minions that are poisoned before hitting max stacks. That's the rough equivalent of getting about 1200 stacks on Nasus' Q in a free lane. That's insane.
I actually was delighted whenever the enemy jungler or top laner was forced to appear in my lane because I could get 10-20 stacks quickly, as opposed to having to farm that out through CS.
Obviously this is a huge outlier, given that it's a 4v5, but the implications of this are simple: You can almost completely shut down Cassiopeia's late game power by not ever getting close enough to her to let her poison you. If you're willing to let her get a free, non-harassed lane, then Cassiopeia will never hit her late game hypercarry status. Fighting Cassiopeia in lane and letting her trade is what will let her stack her passive.
I think the passive should be adjusted so that letting Cassiopeia farm an empty lane is dangerous for the enemy team. Maybe give her two stacks per poisoned minion kill, and decrease the rate of stack gain for putting poison on enemy champions, or something similar.
I think she just feels really bad to play. She's like Anivia in that it's a lot of work for very little reward. You've gotta play really well to put out damage to compete with other ap carries.
I think Riot was just a little too nervous of her becoming a crazy powerhouse so her numbers are definitely on the low side. Especially on her ult, I think that ability more than anything just feels really bad. Really short range for not a whole lot of damage anymore.
It's both.
It's a nerf to her early game and a buff to her late game. They wanted to shift her identity from a lane-bully that punishes positional mistakes (and has the ability to all-in you after she softens you up first) to that of an AP-based hypercarry that bides her time before she can 1v5 the entire enemy team and get a pentakill.
Her laning phase is a lot less safe now and she requires a significant amount of babysitting from her jungler, particularly if the enemy jungler is aware of Cassiopea's hypercarry capabilities and camps mid to prevent her from reaching her potential.
If you thought Cassiopea was team-reliant before (given that you're a short-to-mid-range immobile mage), this is even worse now. Especially in late game, your team needs to protect you because your only form of self-peel is your ult, and it's easy for the enemy to juke.
On the upside, if you manage to farm a couple of items (any two of the following: DFG/Zhonya/Rabadon's/Abyssal/Void Staff), and you have at least 75 stacks, you can now 1v1 anyone (even the enemy ADC) except for assassins or high-burst mages (unless you catch them face-first with your ult). The heal on your E lets you survive the consistent damage of a bruiser, tank, support or ADC, so your only concern is burst (and CC; CC is always a concern for you).
Oh, and managing your mana is now something you need to put some thought into. Don't buy Tear or Chalice; you need to rush your big ticket items as soon as possible, which means that you need to take advantage of your E's ability to restore you mana when it kills something. It must become second nature to you to, after a teamfight, go into the jungle or a big enemy wave, throw down a poison and then E yourself back to full health and full mana. This also means you need to think about last-hitting minions in lane with E (which is a good way to mitigate the damage of harass-heavy opponents like Ziggs, Syndra and Orianna). If you're careful, you will never run out of mana, and once you get 75 stacks (which should be your most immediate goal), you can start using minions as a source of health (much like an ADC with lifesteal).
I don't know if I fall into the category of "fairly informed", but I'll offer my input with a couple considerations; hopefully it'll be another data point to consider along with everyone else's comments. Firstly, I haven't played old Cassiopeia for a loooooong time, and my mechanics have improved significantly since then, which can really affect how I feel on a champion. My only game on new Cassiopeia was vs. bots, and a Swain for that matter, who I think Cassio counters. Overall, I couldn't get the hang of old Cassiopeia last time I played her, but I had a lot of fun on the new Cassiopeia. I think others are right in saying her lane bullying has been reduced significantly; I predict against vs heavy harass lanes you should have a harder time, but you scale really well into late game, and building stacks isn't too difficult. I think you can sort of compare her to Ryze; play safer early and scale into late game where you deal massive DPSs. I don't know if old Cassiopeia players will like her, especially if the reason they played her was for the lane bullying, but she seems like a pretty strong late game champion so far.
For the most part I followed Faker's build on the new Cassiopeia, Tear -> RoA, maxing Q first, E second (Faker has another build available where he maxes E first, though assuming he modified his build accordingly, I assume the former skill order is preferable, since the former is his second game on Cassiopeia, where he modified the skill order). Two things I noticed about maxing Q first; it might cut your damage early game, but you are far less mana hungry, since your E gets very costly before you get a few items. I didn't have any mana problems after the very early game following this build. Also, at 90 mana a pop, I doubt you'll get many Es off unless you have the mana pool to support it (it's also technically more mana efficient at lower levels; 90 mana for 3 Es, 165 + 3(40% AP) vs 1 E, 155 + (60% AP). Of course you have higher DPS with max E, but it you probably won't have the mana pool to support it early game; E spam also helps you stack more quickly). Second, you get to use E more, which means more stacks on your passive. I think I managed the first tier of her passive (75 stacks) at about 5 minutes, and this was me CSing at like, 50% efficiency (had about 25 CS, but you can double proc your stacks by hitting poisoned champs with E). Once you get better at CSing with poison and with E, you should be in really good shape in terms of your stacks early game.
Overall, I don't know if this can conclusively be called a nerf or buff; certain aspects of her were nerfed, and others were buffed, resulting in a pretty different playstyle. She seems able to harass early game, but don't expect to get lvl 2 solo kills like you can on some champions. I think her overall playstyle feels a lot like Ryze; you deal more DPS than burst, so you can lightly harass fairly well, but you're probably not going to all in pre lvl 6 unless you're really confident. Also, you don't have to chase kills, just like Ryze (though you can, since you have bonus MS and a slow), where if you scare them off just so you can CS and stack your passive more, you just become more and more difficult for others to deal with later in the game. Looking at your stacks; tier 1 basically gives you a bit more AP and spell vamp, tier 2 is basically an Athene's or Morellonomicon, but without the mana regen (which you get from other items), and tier 3 is basically a deathcap. Fully stacked, you basically have an extra two items late game; DPS with her E is pretty devastating late game, and also gives her really good farming through most of the game (it's sort of an extra auto attack; again, she's more geared towards DPS than burst). Ultimately, I think she's strong right now, but she plays a lot differently than she used to.
Thanks guys this gives me some good insight, I mean I played it a few times and it was once I got rolling that I noticed its very very strong but that was also her prior to this.
