I've booked through Priceline several times. It's worked well when I've done it. Never tried it for Orlando though.
I check the prices via Orbitz first (they have a cool matrix thing so you see the prices for all the companies at once) and then go to Priceline and try and undercut it by a significant amount. Sometimes it works, sometimes it doesn't. If it doesn't accept the price I offer -- then I go and book the regular way.
Never had any problem with picking up the car at the destination airport.
Hope this helps.