I've played a fair bit on iOS. The important thing is to write the game for the touch screen, not just make a traditional console game and expect it to work.
There is some official ports of the various Sonic games (Master system, Megadrive), and a port of Rayman 2 (Dreamcast). They play OK. Just OK, not great. Since you can't feel the buttons on the screen, you don't know fo sure where they are, which is super-important in most games, especially platformers. You also can't rest your thumb on the button, since that would registar as a button-press, so you need to hover the thumb over the surface, losing spatial and lateral precision.
Two racers I know were developed for touchscreens are Death Rally and Reckless Racing. They both play GREAT! (With capital letters and exclamation point.) The steering is centered around your finger each time you touch down on the screen, so you just need to swipe by diffrent amounts, not hit some exact spot. Swiping gives you tactile feedback since you can feel the friction. If you just swipe a very short distance, you can even roll the thumb instead of dragging it against the screen, so you know exactly how far you move.
The kind of game where touchscreens shine are the more "casual" type without twitch reaction and pure reflexes. Preferably you should be able to interact with the game screen "directly", without any controller-buttons. Stuff like Bejeweled, Sim City, and Angry Birds are great examples of this.
Personally I think Angry Birds suck as a game, but I guess that's a matter of taste.
I'm not much of a fan of FPS:es outside the PC, with a mouse. But the few I've tried on touchscreen have worked at least as well as what I've played on consoles.