I just got a great deal on a mini van mid January from Alamo through Priceline by bidding. I checked all the sites, discount coupons and codes, then started by bidding at $28 a day. That was rejected, but I was invited to bid again 2 days later. I bid $30 a day and it was accepted. After taxes and fees it came out to be about $305 for the week.
I figured I saved about $100 over the next best deal and more than half off the Alamo website. The only drawback is that I cannot change or cancel the reservation, but I won't need to.
I've rented cars through Priceline many times and never been disappointed.