Has anyone tried Bookhog and if so how have your results been?

www.bookhog.com I just received a card in the PTO pack that says you can register your school for free and you get 10% back of any books purchased from their site. It says they send you posters & flyers to send home and that there is no cost involved. It seems like you lose nothing for trying it, but I would like to hear how anyone else has done with them.