I've had mine on since day one (24th June), and unfortunately, it has put at least 5-6 minor scratches/marks in the metal. And I take it off every couple of days and clean it quite thoroughly, and make sure I never put the phone anywhere with too much dirt in it. I do put it in my pocket though, but always with nothing else in it, and I'm fairly sure there's no sand in my pockets, or anything else that abrasive.
All initially seemed ok for a couple of weeks, then one mark appeared near the speaker. Then, again, for a few weeks no further marks appeared (it's after the first mark that I started cleaning it every couple of days), and when I had a look just now, there was something like 5-6 new marks, all over, bottom, top and both sides. 🙁 Very disappointed.