I have a few micro scratches on the back of my X, but those are just unavoidable from taking it in and out of my pocket. So far no actual deep/noticeable scratches.
The glass backing would show more scratches over 7000 series aluminum in most cases. But I'm also willing to believe that the majority will be using cases to protect the back of their iPhones, which most scratches would occur for those not using cases.
However, the concern to me, is not necessarily the scratching, but the shattering affect with the glass if it drops for those who do not use a case.