My wife's on there, but she's had a few "choice" experiences with some members. For those who aren't familiar, BookMooch works on a points system. You get a point for sending a book (more for international, plus partial points for listing books) and it costs a point to receive a book. If the book arrives in poor condition, it basically goes for free (sender doesn't get a point, recipient doesn't spend one).
Well, my wife sent a book and the recipient reported it as a bad shipment because the envelope was damaged (not the book). After asking other members, she tried to get her point from the admins since the book itself was undamaged. They said it was out of their hands. Two weeks later, the same person asked my wife for another book. My wife declined the request, not wanting to chance the same thing happening again. However, the person reported my wife and she lost another point for rejecting the request. So, one book sent and, instead of gaining a point, she lost one...and the recipient got a perfectly acceptable book for free. My wife has since not used BookMooch nearly as much because it seems like she's just getting burnt by other members.
I don't use it because the books I read aren't the kind that are normally the "pass it on" kind (try finding any of Nietzsche's critical editions in German).