Personally I feel that the brushed back on the M8 seems a little....urm... cheap. Then again ive only seen it in photos. I feel like the metal that they use is going to be a bit soft, like the aluminum that the iPads use.
It would bother me alot if these scratches started happening out of everyday use. Coming from my Q10 and Z10 i dont really worry much for scratches because of the glass weave on the Q10 and the rubber on the Z10 but on metal it just seems like it would be more noticeable.
This phone does not look, feel or in any way, shape or form fit under the description of "cheap." It's up and away the nicest phone I've ever owned or even seen. No scratches at all for me.