I've used them a couple of times. If you don't want to book directly on their site, they list the discount codes they used to get their price at the bottom of the search page. You can plug those in directly, on the rental company's website. We haven't had any trouble, though we used it at LAX and again once in San Diego.
This site is great! I just saved $200 on my rental for December. Woohoo!many times I love them