Not a fan of them. They seem to be missing the backplate about 99% of the time, so you have to to on ebay and buy a new one for $15 or $20, probably from the same jerkoff who stole it in the first place. That by itself is not worth the pain in the ass, though the other parts like cables and screws are easy.
Also, sometimes you do get boards that were returned for a reason. I did end up with one where the LAN adapter was intermittently not working and then crapped out entirely.
So basically, no, I wouldn't buy a motherboard open box. Other components yes, but not a board.