I personally have been a member since last year. I've loved it. It's so much cheaper than buying the books and you can keep books as long as you wish. Once in your hands, it is yours and you do not have to return the book like you do at the library. Now, the books do have to be in "good" condition which is defined more specifically on the site, but generally means that the cover is intact, there are no stains on the pages and no water damage.
It's a great place to try out and you get 2 free credits once you list 10 books. Happy swapping!