I have just finished updating my iPhone 4 to iOS 6. I've been running 6 on my iPad 2 for a while now and thought it was time to update. I prepared the phone for the update by clearing enough space. I also did the update through iTunes, rather than over the air.
Having been made aware of the potential problems after updating, I checked everything I could think of after doing the update.
Photographs. A slight glitch. Initially, the phone reported that there were no photographs in Camera Roll, but after a few seconds, it restored 573 photographs.
Wifi connectivity. Works with no need to mess with Settings.
Bookmarks. All present and correct, and automatically added bookmarks from iPad to iPhone.
Music. All present and correct.
Email. 3 accounts working. VIP list transferred from iPad automatically.
Safari. Incredibly fast download speeds compared to 5.1.1.
FaceTime/iMessage. Working OK.
I don't think I've missed anything. The update did exactly what it was supposed to, with no data loss or problems.