I use them every year. Most times (not always-depends on the location and time of year/events going on) you can save quite a bit by doing the "name your own price" option over published rates. But you need to find out what the going rates are first for a given location so you can place a reasonable bid. They also put up what recent offers have been accepted by the rental car companies at a given location so you have a reasonable idea of what they will accept. Also their price usually includes all the taxes and fees.
If you place a bid and it is rejected, you get one more try to adjust the price. If that is rejected, then you will need to change the size of car or location before you can bid again. Also your credit card will be charged immediately if they accept an offer. But I have never had an issue when I got to the rental counter about the charge to Priceline.
Well I guess I got screwed on my first experience with Priceline. Placed a bid for a rental car last weekend. First bid not successful so they gave me a second chance. That bid wasn't successful either so I got the option of booking at a price $10/day more than my bid, which I did because it seemed like a better price than I had found elsewhere. In the end, it was the same price I had found directly with the rental car company because Priceline did not include the taxes initially. I live and learn. To add insult to injury, today's price directly with the rental car company is half of what I paid through Priceline 4 days ago. Priceline already has my money so no cancellation But if I had booked directly with the rental car company I could have ignored that booking and booked today at half price with no penalty. This is why I never deal with third parties.