First off, BACK UP !
Anytime you do a software update on anything you should have what you want to keep backed up. Apple is no different.
I've heard a lot of people touting how they were able to update via wifi and had no issues, that's fine, but it takes 2.9gb of open storage to allow that to happen (4/4s). If you are close to maxed while it is updating, it can cause problems. it's swapping files, give and take, needs room (memory) to do this
I updated 4 iThings Saturday, one of the iPhone 4's bricked, hard boot ( both buttons to shut it down ) then default install of iOS it's a new phone, then DL the backup and it works like a champ.
I down loaded thru iTunes/USB, all but the ATV, took about 30 min after backup don't know why the one phone had issues, but it's fixed. Doing the update thru iTunes it didn't matter how much memory as it used the computer (Mac Mini) as a buffer. Wife's phone had less than 400mb memory left before the update.
It's new and different, I'm still trying to figure out all the stuff, but everything seems to work better. Don't really care for the cartoon look, but whatever.
Our iPhone 4's were sucking down battery ( mine always has ) a few app changes ( a lot of apps are running even though they aren't on the screen ) and it's better.