I've looked into and even bid on Priceline for car rentals, but found that it was rejecting bids equal to what was available for booking on the internet. I just couldn't get enough savings through Priceline to justify the hassle.
I'm a big fan of Priceline for hotels, but I didn't have luck with cars. May depend on time frame, though. Check out biddingfortravel.com for FAQ and recent prices.