Games I've enjoyed on my iPad:
BIT.TRIP BEAT -- Probably my favorite game on the iOS, think pong as an on-rail shooter, with visually and aurally stunning gameplay cues. This is a must have for anyone, and receives my highest recommendation.
Karoshi -- A platformer puzzler where you.. Killed yourself. =P
Spirit -- As far as I comprehended the game, your objective is to create warpholes to get rid of the enemies on screen. Visually looks like Geometry Wars.
Violet Storm -- A direct rip-off of geometry war, but still quite fun.
Geometry Wars -- Needs no introduction, but basically it's a free-roam shoot-em-up.
PewPew -- Read above.
Forget.Me.Not -- Pac-man esque game with paying homage to Atari graphics, unlike pac-man, there's a variety of enemies and level designs, and is generally more abstract.
Game Dev Story (by Kairosoft) -- My first Kairosoft game, and a time-sink at that. The game involves you micro-managing the expenses of games, so involves you spending cash on employees, developing their skills, advert campaigns, getting displays for gaming conventions, coordinating genres, keeping morale high in the office, going to video game awards ceremonies and the launch of new hardwares.. This game pays homage to alot of consoles and games, and has a cute isometric presentation.
Spell Sword -- Part action game, part collectathon, the game is fairly simple and a fun arcadey experience.
Will's World -- If anyone remembers Chip's Challenge, Will's World is basically Chip's Challenge with added options and stages.
Hex Defense -- This game is your typical tower defense game, with Geometry Wars graphics (which is appealing to me)
DoDonPachi Resurrection HD Lite -- although I didn't play the full game (it's expensive for an iOS game), it's alot of fun going through the demo of this game multiple times, if you don't know already, it's an on-rails bullet-hell.