I haven't compared the two on how many hits I've received, but they both seem comparable. Prices seem to be the same. I have found that they find books quickly, but expect to pay a fair sum for the service. Most of the other OP sites have you buy the books from the dealer, this site you buy the books directly from Alibris. This probably would make it easier if you planned to buy more than one book.
Site looks good.
Prices are extremely reasonable. The site lets you search online for a book and then lists used bookstores which have the book. They then give you the bookstore's phone number and address. When I used it, I simply called the bookstore and ordered the book with a credit card over the phone. You must register to secure a password before searching.