i opted out on any shooters other than the cave ones. Even though they are designed for the low res iphone(i only have an ipad), i thought getting the cave games for 10bucks was a fair deal(perhaps not to most iOS users though). I've got a ton of other non shooting games so I have plenty of apps to keep me busy.
I tried geometry wars HD but I don't think the control scheme on the IPad works so hot.
I've read about some good Iphone shmups, but partly because I have an IPad and never had an iphone I don't want the lower res games.
I can't play DFK on the ipad anymore though, I want to wait for the 360 release. I've played the game enough to imagine what it will look like on a larger screen, and to play with a Semistu stick and not my manstick.
I'm interested in hearing about what others found fun as well, particularly if its an Ipad app