Have used priceline TONS in the past for hotels and 2 times for rentals. Both times have been fine, and well below posted rates. I wanted a convertable or SUV this time, but they were too $$ for me, so settled for a car. We're down in march break, so the rates were higher than last time I rented, but still well below retail I believe. There is a great site biddingfortravel.com that can tell you what other people have gotten in terms of rates, and it has tons of info on bidding strategies. It depends on time of year what the rate will be, but I would say start early and low, and then gradually work your way up towards your max. Start at 50% of what you see posted on other sites, if not slightly less, and work up from there in $2-$3 dollar increments. You can re-bid right away by changing the type of vehicle you are bidding on, otherwise you have to wait 48hours to try again, so starting early gives you lots of time to play. Good luck!