The touchscreen is the controller for IOS devices, so whenever a game can use it well, you've generally got a solid gameplay mechanic. Gasketball is one of the latest games to hit the Appstore that uses the touchscreen in an interesting way. Players try to make baskets by flicking the ball onscreen. You follow the computer's lead and try to recreat its shots. Sounds easy enough, right? Throw in some teleportation portals or other gadgetry and you'll wish this was just a friendly game of horse. The mechanic works well enough and is fun; but I found myself wishing the physics of "throwing" the ball could be a little smoother and more accurate. Many times the game just failed to read my shot.
Gasketball is free and recently updated with even more free levels.