Has anyone ever used priceline.com to get a rental car?

I wanted to get a rental car by naming my own price on priceline.com and I was just wondering how I should go about it. Is it beter to wait until closer to my travel date? Do you get a lower price that way? Any info would be great.Has anyone ever used priceline.com to get a rental car?
I used priceline about 3 weeks before my trip and got a car for about $18 per day, where the cheapest on travel.yahoo.com was $25 per day for the same area. So I think I got a decent price. I can't say if it would have been much better if I waited...I guess it depends on the area and how many cars there are. In Denver airport there seemed to be a ton of rental cars. Once I was at L.A. airport and there were very few rental cars.