After the WWDC '08 Keynote, everyone wanted this iPhone game -- it received almost as much hype as the iPhone itself. Super Monkey Ball from SEGA definitely has some good qualities, however it does have some bad qualities as well. First off, this game costs $9.99 -- a little steep at first glance, but totally worth it after you notice that it comes with 100+ levels.
The graphics are beautiful, and it definitely pushes the system, but I do have some complaints about the game. First off, the game is extremely hard to control with the tilt-sensors -- perhaps an update with some refined calibration is in order. A bigger complaint about the game: when you have the silent switch flipped on (meaning that the phone shouldn't make ANY noise), the game overrides this and plays the sounds anyway. So, be sure to change the sound settings before you start trying to play this game in a very quiet library or doctor's office. You also cannot pause the game, go to the home screen, and reopen it to start playing again (this might be an SDK issue, rather than a specific SMB problem).
Overall this is a solid game with good performance/graphics. While you won't be able to play this game in the car, it still provides hours of fun sitting extremely still so as not to anger the tilt-sensors. This game is available for $9.99 from the App Store.