I agree with Nick in that many of my most-used and most-beloved iPhone and iPad apps are the ones which look and feel like they were made by Apple. A good rule of thumb is, when in doubt, use the same UI design found in Apple’s native apps. If you are going to do something custom then have a good reason why and do it better than Apple would do it.

See also this article from Marco Arment on optimal iPhone UI.

Nick Schaden on iOS App Design