I haven't tried Priceline but I usually go on Kayak because they compare all the websites including the local rental companies. We need a minivan and we ended up going directly through Carl's Van Rentals to reserve ours. You can easily Google codes to help you save on your rental but it is a lot of trial and error to see what will work with your dates/car type. Don't forget military/USAA/AAA etc. if you qualify for their discounts.
They have discount coupons that can have substantial savings. Be sure to try all of them....some are limited to dates, some to regions, etc, so some may not work, some may have small discounts, but I've always found good discounts here.
I have no experience with priceline.....much as I want to save money, I'm just not comfortable not knowing all the details before I spend my money, lol.