I used Priceline in May for our trip to WDW. I started my bidding at $10 and ended up with $13 a day plus tax and fees on a midsize. Our rental was with Hertz, we used the shuttle to get to Hertz, when there I used the kiosk to pick up the car. With a Priceline reservation there is no need to wait in line, the kiosks work for these ressies.
For the PP, once you have entered you bid with your information, if that price is not accepted nothing is charged to you. However if you submit a bid and that is accepted, the full price of the reservation is charged to you and not refundable.