There were all kinds of problems for many users yesterday because Apple's servers were overloaded, but things are reportedly much, much better now.

Personally, my iPad upgraded without a problem.

On the other hand, I've had tons of problems trying to get the iPhone to upgrade, mostly getting error -42 (out of space, I think), maybe because the iOS5 takes up a lot more space, which is leaving me without enough space to do the restore after the upgrade. Still working on finishing that upgrade (iOS itself is upgraded, but I'm trying to restore my content). Beware to clear enough space on your iPhone/iPad before doing the upgrade.