I've been swapping books at Paperbackswap for a few years - it works really well for me! I get books from my wish list pretty regularly, and when I'm done reading a book I can swap it. Before I found PBS, I didn't have a good option for what to do with all the books I had read. The site allows me to turn them into more books, very easily! The best part is the feeling when someone else in the club wants the book I have - it feels so much better than selling them to half-price books. A couple of months ago, the site went to a membership model - but the cost is pretty small (20$ or 12$ a year), and if you don't pay the yearly fee, you can pay-per-book if you want. 49 cents a book request is OK with me, considering that some of the books I get are brand-new hardcovers. Yes I loved when it was free, but how could that last forever? I am not wasteful with money, but I believe in paying a reasonable cost for a service I find useful. It may not work for everyone, but Paperbackswap is definitely worth it for me. Any serious reader should check it out.
PS An earlier review said that there was no notice of the site change to membership model, but I got an email telling me about it and there was a notice at the top of the site that (if you click it) gives more details. I guess if you missed the email and missed the notice and didn't read any of the Discussion Forums it would have come as a surprise, but the change didn't happen without warning.