I actually like the changes, I played Cassiopeia a bit before the rework but couldn't like her, she felt a bit Shaco-ish, as in, I MUST get fed off the early game because I simply don't have the tools to be strong lategame. Now while I've been completely unable to make her work (it's a player skill thing, she went from being hard, to being HARD), the idea and the promise of a lategame monster are really interesting.
My main problem with the pre - 4.21 Cass was that she was forced to push the lane in order to farm stacks, which was awful, but with this change, stacks just come naturally, and even if I'm unable to pressure the enemy laner and poison him, the skirmishes and teamfights that are bound to happen are enough to get me a decent amount.
Overall, I may be biased by having played old Cass and not liked her one bit, but I feel this rework really brings life to the champion, then again, I believe the numbers to be a bit undertuned, specially as it concerns her lategame, but that's the kind of tuning that these updates (like 4.21) are meant to achieve.