I always have had great luck with priceline...BUT you've got to wait until you are about 2 weeks (or less) from travel time. Before then, the rates aren't that great.
Here's what I do. I make my best "fall back" reservation using codes (mousesavers, costco, whatever) when I make my other reservations. I'm not required to put in a credit card, or pay in any way. I use this reservation if priceline bombs for me (which it has, albeit rarely). If I get a good priceline deal, I cancel my fall back reservation.
Then, when I get about 2 weeks out, I start pricelining bids. One thing I've noticed recently (booking my most recent trip which is 10/10-10/17) is that the CHEAPEST priceline vehicles are full size ones. Compact and economy bids at $13-15 per day drew the response that I was bidding too low, and likely my bid would fail. However, if I put in $13 for a full size, it would put it through (although it was ultimately rejected). I got a full size last night (13 days before our trip) for $16 per day (not including taxes and fees). My total for 7 days with taxes and fees was 181...the car is from Alamo, so it's right on site and very convenient for us. After I got the car, I went on Alamo's site, and the best I could do for the same week, same car was $546...obviously a TON more.
Priceline works great if you are patient and understand a little about bid strategy. I'm by no means an expert, but I've always gotten cars for a great price.
ETA: I wanted to be clear that when I say priceline...I mean the part of priceline that lets you name your own price. The "regular" pricing section of priceline isn't anything special.