Thought it was worth mentioning that World of Tanks is now on iOS. No artillery, and it seems like the light tanks stop developing at some point, but pretty cool. I'm really impressed that they got it to work, since I know it's a power hog on regular computers. I really like that it should level the playing field since everyone on iOS has basically the same (or a small, known set of) hardware, so the game will work basically the same on everyone's machine.
Plundernaughts - a space-themed pirate ship battler- is also fun, though frustrating because of the IAP setup (you must spend your rare resources to make more than 6 attacks per hour). Ed: I've stopped playing because of the IAP setup