So far I've sent out 5 books. I send them all media mail, so the cost to me is minimal, and I have received 3 books for free. The extra points I have right now are waiting to be spent on books that are on my wishlist which is growing every day! When a book on your wishlist becomes available you receive an email letting you know.
It's a great site. Definitely would recommend checking it out if you have some unwanted books laying around!