I was an active member there, and now dislike the site. It has gotten so much press and hype that so many people joined. Now there is a tremendous amount of members. At the start it was very easy to get the books that you want. Now it seems nearly impossible because there are so many people. Since it's a first come first serve, those of us that can't hang out on the site continuously don't get any chance to get the books. And then there are a lot of new members that have books that I request, only to have them decide the site's not for them and cancel the request. Too much of these things are happening there, so I have been very turned off by the site.
I signed up for PaperBackSwap.Com and started getting books that I had on my wish list at BookMooch for over a year. You get "in line" so to speak for the book. So when the book is available, the person that is next in line gets 48 hours to decide whether they want to book. If they don't have the points for it, they can buy them, and the money goes towards paying for the running of the site. They also stress how important it is to use the ISBN to post your books. Too many times on BookMooch people don't know what they are doing and you end up requesting something that is posted as the hardcover, only to get the mass market edition.
There are good things about BookMooch, like being able to request a book that someone lists the condition, instead of just getting whatever comes because you are next in line. But I prefer the system at PaperBackSwap.