Don't like that message in iTunes that keeps coming up when a new OS update is available.
I've kept updated for the most part with OS 3.0. I've downraded a couple of times back to 2.2.1 at times when I needed my iPhone jailbroken or because apps were not OS 3.0 compatible.
My biggest reason for updating is my desire for iPhone stability. Dropped phone calls are bad enough. But unacceptable data rates are something else. Since the days of OS 2.0, Apple seems to be focusing more on stability. A case in point is the safari browser. Very rarely do I have Safari crash with 3.0.