I debating using mine without a case? Yes, I know about skins and have used them in the past. I want to use the phone with nothing on it but don't know of its durability.
It can and will scratch if it comes into contact with the usual suspects (keys, change in pockets, etc) ... If you're careful and haven't had trouble with past phones go for it, but use a case/skin if you want it looking new.
Case it. I've seen other people's aluminum phones after they use them naked for a year and it ain't pretty lol. I just use clear cases, preserves the good looks without sacrificing protection.