I've used them a number of times (3) and actually taken the flights- all to Europe, received FF miles on UA.

I've probably "won" a total of 6-7 auctions. 2 couldn't be booked for the dates I wanted, and refunds were promptly given. I think I've "walked" from 2 or 3 others, with no repercussions, although they state fees, blah blah blah for no shows.

Yeah, you've gotta watch the fine print and know the pricess available elsewhere- AS WITH ANY PURCHASE!