For older iPhone users like me (I have an iPhone 3G), the recent software upgrade (iOS 4) has been a disaster. The upgraded operating system brought only a few useful features to the older 3G and 3GS phones, and along with the minor upgrades, severe performance degradation. Frankly, it's just awful. The biggest thing is a major slowdown across the board for all apps, to the point that I use the iPhone less than I used to because it is painful.
But perhaps there is hope. An iPhone user reports getting a short reply back from Steve Jobs that promises an fix "soon." I can't wait.