I upgraded my iPhone 3 GS to OS 4. Not as smooth as I had hoped.
First, the backup took 14 hours and 30 minutes. Then after the upgrade the phone was no longer recognized by Windows. I ended up having to uninstall iTunes and all it’s pieces, and reinstall. Then the phone synched.
About the only thing I notice different is that there’s wallpaper under the AP buttons. Oh, and all my ring tones went away, and my blue tooth connections.