Clearly Apple sent this one to the presses a little prematurely. I've noticed a lot of people have been having problems. I have two devices, iPhone 3GS and iPod Touch 3G - iPhone is my main device, so I held off and decided to upgrade on my Touch only.

Didn't have any problems per say... however my device appeared to be finished with the upgrade before my iTunes. Usually when doing any kind of upgrade or sync, the sync screen pops up and the device isn't useable until iTunes finishes. With the iOS 5 upgrade, my iPod fell out of sync mode into the set-up phase. After finishing the set-up (asks to turn on location services, and enter your Apple ID etc) my device went right to Springboard, and appeared to be %100 useable. Except my iTunes indicated it was still configuring apps. I can see someone prematurely unplugging their device thinking the upgrade is complete, and having something screw up because iTunes wasn't finished.

Not sure what the reason for this was - if maybe the iTunes interface was lagging.. But I've never been able to use my device while iTunes was doing anything, especially an upgrade or a sync. So if you haven't updated yet, make sure your iTunes has completely finished doing everything before unplugging your device.