We used this for our trip last summer, after finding very few "deals" with the major companies. I was a little worried at first, having never used Priceline before, but can honestly say that it was one of the most gratfying parts of our trip.
I bid $18.00 per day for a full sized car, fully expecting it to be rejected. (I had a back-up, fully cancellable ressie w/ Alamo, just in case!) It was accepted by Hertz, and we were extremely pleased. We were even more pleased when we were upgraded to an SUV (a Mitsubishi Montero), as they had run out of full-sized cars. What a deal!
I'm not sure if this type of deal is always available. I do know that there are often more cars than customers in Florida, and that the companies will compete for your business -- you just have to know how to play the game.