Hi, welcome to another "Is x worth buying?" thread by me. I'm an Unranked, I don't wanna play ranked due to toxicity. I'm currently trying to play mages at middle lane. I liked 
's Q/W+EEEEEEEEEEEEE nature and her strength even when behind. I'm planning to buy her, is she really worth buying? Also can you give me some tips?
Cass is a lot of fun. I find I can take her mid/top/sup. Check out Bjergsen's video on lolclass for some tips. http://lolclass.com/video/96/cassiopeia-basic-champion-guide-with-bjergsen
Also, check out youtube videos with Voyboy playing Cass, he gives some good tips.
She definitely is really fun, but some people detest her reworked passive and E spam. She is VERY powerful lategame and even has a somewhat good early game. Check out Voyboy if you want good tips. (There's a bug where if you flash towards your enemy and aim your ult backwards, it still hits the enemy :).
I'd say expanding your champion pool to different things is never a bad idea. Cassiopeia is a very strong mid laner, despite nerfs, the only thing they've accomplished is reducing a bit of her early bullying potential.
There's an issue with most "mages" in mid lane currently, as mid is typically ruled by assassins, it's workable, but can be a bit more difficult to pull off. I'm a big fan of Brand, Cassiopeia, Viktor, and a couple of your other standard mages that can be played in mid lane, but they do tend to struggle and require you to play a bit...differently than you typically would against another standard mage. I think mobility is the largest contributor to this problem, where most of these mages lack any real form of mobility. Cassiopeia had some worked into her kit with the rework, I believe it's from landing her Q, but other than that she has to ult to escape.
I personally feel that these type of mages are really good to learn, because they also teach you proper warding, as they're easy to gank, and have a difficult time escaping ganks, you're forced to ward up really well in order to make certain you don't get crushed by frequent ganks, which will happen easily.
The good news is that of these mages, Cassiopeia is a very good one to have in that situation, proper skill usage can easily turn that gank into a double kill because of the amount of damage she throws out. She's got a very high difficulty rating for a reason, but learning her is extremely rewarding.
As far as tips, the biggest tips I could provide, as I'm admittedly still learning her myself, is to practice her a ton and don't get discouraged when you lose, it's bound to happen a good bit while you learn her. Don't forget to pick up a lot of wards, I always have a pink down and leave the fountain with at least 2 stealth wards as well, you need to see the ganks coming in advance or you can't back off properly to save yourself. Finally, unless you can definitely kill the person, don't get too carried away with spamming Twin Fang(E), it will rapidly obliterate your mana pool and leave you in a terrible position. Remember that killing targets with Twin Fang will refund it's cost + 3% of your maximum mana, which is why it's important to get the kills off of twin fang be it on minions or a champion. Finally, hitting targets with your ultimate does more than just the CC, it also provides a substantial chunk of damage.
Do it. Go for early sustain runes/masteries, spread more into tank/utility. Mana to start isn't bad, focus on gold, and remember her strength is in zoning the opponents. Know your matchups and pay attention to the summoner spell choice of your lane (lets you know what he's about). Takes alot of micro and can be stressing, but can be rewarding if protection from the right comp. Nomicon and Mask are mandatory.
