So I played the first couple levels with a buddy of mine. Initial thoughts:
My friend likes Cap a lot. Haven't gotten a chance to play with him, but I'm told he rocks.
Thor is hardcore.
Spider-Man and Wolverine are about what you'd expect, and moderately useful/cool.
Luke Cage is an uninspired waste of time to play with. Thing isn't much better. Colossus was decent because he could use Ultron bots like rag doll weapons, but he died on us a couple times. Generally, we didn't really like any of the bruisers except Thor.
I'm partial to anyone that flies, so I like Ms. Marvel and Iron Man. Nothing special though.
I kept thinking of the unholy monster Iceman was in X-Men Legends, so it was hard playing with him sans a bunch of upgrades.
Deadpool is decent. Dialogue could be slightly better, but "I'm the best at....whatever it is that Wolverine does!" was priceless.
...
All in all, a nice fanboy-romp through Marvel so far. Nothing special though.