BookMooch

I love books, and I cannot throw away an actual real book, even if it's quite bad. Right now, we tend to drop off the "oops, this wasn't good" and "how did we end up with three copies of this" books at our favorite coffee shops (because a good coffee shop should have a bookshelf full of books). But emzebel mentioned that she uses BookMooch, which is a site for trading books. You only pay for shipping, which is pretty darn cheap for media mail. Has anyone used it? Any comments?