I've had my space gray MBP since launch day and so far there has been no discernible wear or scuffing on the palm rests (where the most friction typically occurs).
I did clumsily drop the computer (while it was closed) on tile from about four feet, and you can see the exposed silver on a corner where it's dented. But the scratch/dent needs to be very deep, as I can only see space gray along the "spine" of the computer on the other end where it was scratched.
I went in store today to look at the silver and gray MacBooks to make a decision. I went in thinking I'd prefer the gray but ended up choosing silver. The silver color looked classier to me when contrasted with the black keyboard and display bezel. The gray was too light-colored in my opinion, and didn't match the keyboard and bezels well. Had Apple gone with a much darker shade of "space gray" I would've gone for it. But the gray they chose I think looks kinda cheap.
I would recommend everyone look in store to make a decision. The images Apple has up on its website make the space gray look a lot darker than it actually is. (Unless they went with a darker shade than the MacBook that I don't know about?)