I started going naked, but then quickly picked up micro scratches on the back glass portions and tiny chips on the beveled edges. It's one of those things where you don't notice until you look carefully. I went back to my Caudabe Veil case in a hurry. It feels just about naked with the Veil anyway.
Mine spends most of it's time out of a case and its holding up great. Bar one small mark on the bottom where I dropped it onto a glass table it's practically brand new looking. It's only got to last until the 6 is available and then it's already sold. Just got to hope I don't damage it between now and then (touches wood).
I use my 5S without a case most of the time. It took a spill out of a moving golf cart onto a really terribly paved golf cart and picked up some nicks and dings in the corners. I've also had quite a few micro scratches on the display which I never had an issue with on previous iPhones - why I'm hoping for some sapphire display for the 6.
Mine is starting to sort of chip at the edges and a few small scratches on the back. It's been dropped by my toddler so now I use a case at home and all weekend. And case-less on weekdays at work. Sort of the best of both worlds.
I little nicks here and there and i have kept it in a mophie case....every year this happens. I keep it in the case all year and resell when the new one comes out and every year i get little nicks in the aluminum. mophie needs to get their act together.