No case. I've never used a case on any of my phones, but I also am not prone to dropping, don't bring it skiing, typically carry a bag rather than have it in my pocket...
I have dropped it once though. It has a nice little dent right on the corner. No glass breakage. I have insurance on it through Apple though, so I can replace the phone entirely up to 2 times as long as I have a few pieces of it left.