I've had 5 (black) K&M hangers on a wall now for 15 years, and they remain rock solid. They've kept my guitars both safely off the floor, but always in easy reach and ready to play, and that's really all I need them for. The do look good too.
I have noticed that a small amount of black residue on the necks of a couple of my guitars from the rubber sleeves on the arms of the hanger. This is no big deal, as it just wipes right off very easily.
However, I got a guitar with a Nitro finish, and I'd heard that this might be more of a problem, so I switched one of the original black hangers for this white one, which is claimed to be nitro safe. Well, its been on the wall now for 6 months (other than when its being played, of course!), and absolutely no marking, marring or damage to the guitar. So job done.
If I had a minor complaint, its only that you can't get this silicon sleeving on an otherwise black hanger, so I have one that stands out a bit.