Played PBE Cass
(I'm crossposting this from the PBE forums because this seems like a better way to get a discussion going, and Stashu is responding more over here than over there.)
Right, so first off, the little 'horns' on the middle HUD completely block off the number of stacks I have if that's the only buff I have that goes there (so unless I'm using a potion or am currently skilling up). It's really annoying and you need to change that ASAP.
Anyway, so I played three games. In terms of power, Cass seems okay (but, again, PBE, I might have simply been better than my opponent.) I won all three games and I can safely say I contributed somewhat in all three games (though the third game was a 4v5, so I don't really count that.) I ACTUALLY managed to bully someone out of lane (it was amazing, I watched them back with glee!), and I felt that I could go RQWE and do decently with the new passive on E. So if this came out on live, barring me not noticing something huge, I think I'd be fine...ish.
My big three pain points: One, you are constantly starved for mana. After a teamfight, I'm OOM. Trying to solo a buff camp before Rabadon's, OOM. Trying to take dragon/baron, OOM. I feel that Cass constantly needs blue buff or something (because all of the big mana regen items give large amounts of CDR, they're largely wasted stats, plus Chalice is pretty weak now as is.) because I was OOM way too much. Her mana is fine in lane, when she constantly has minions to last hit with E so she keeps her mana up, but in every other situation, I wished I had more mana. I got a RoA in the last game and I THINK that might have been enough, but I didn't do many objectives or teamfights, so I couldn't tell.
Secondly...that passive. Did you know, when I couldn't see the number of stacks I had, I didn't notice it at all? Like, I occasionally saw the animation for reaching a new milestone, and the icon occasionally changed, but all of the buff things it offered were so small I didn't notice. Like, in the second game, I was suddenly clearing jungle camps in two or three E casts instead of 6 or 7 and I went 'well it appears I've turned on', probably because I got a Rabadon's at the same time, but before then, I might as well not have had a passive. Like, it didn't feel like chains or anything anymore, but I simply didn't feel like it mattered until suddenly it did.
Third, I didn't have waveclear. Like, even in the games where I maxed Q first, I couldn't clear the casters in one Q cast, I had to E each individually, let alone the melee or cannon. In order to waveclear at any speed, I needed to cast far more spells than most other mages, and because of that, I had to use more mana which just tied into my mana woes.
So if I had to give two pieces of advice, it'd be to figure out a way for her to be able to clear jungle camps or waveclear without losing all of her mana, and work on her passive some more, because wow that's underwhelming. My personal idea is to have a new passive, where the amount of poison damage she deals to an enemy is proportional to the amount of mana she gets back if she lasts hits it with E, allows her to clear jungle camps for extra farm and not to constantly be draining mana when she tries to push a lane.
Again, three games on a PBE environment does not a good sample size make, but it's after midnight here and I'm tired. So this'll have to do for now. I think you're getting close, Stashu, but that passive is really being a drag, and I am having worse mana issues than Anivia.