I upgraded my 32GB 3GS to 4.0, and had two minor glitches.

The painful one was that it stopped working with my iPhone-ready car stereo (Alpine x305 head unit). However, the fix for that was to reboot (hold down power and menu buttons for 10 seconds) while the iPhone was plugged in and the car stereo was on. No idea why, but that fixes it.

Much more minor, but I also had to repair my bluetooth headset.

Other than that, it was painless, and app folders and the unified inbox are nice improvements.

Bob