I went from backpack to panniers, and then back to backpack. I've been using a backpack exclusively for over a year now.

I'll never go back to a rack and panniers for bike commuting. It's just too much of hassle, having to move my belongings back to a backpack on days that I ride the bus to work, and then to move them back to panniers on the days that I cycle in to work. Now I just grab my backpack, no matter how I'm getting to work.

It does help a lot to have a very good cycling backpack though. If I were using some cheapo WalMart backpack, I'd probably be miserable.