I've used priceline numerous times over the last few years. Always for hotel rooms. I've tried bidding on car rentals but was never successful.
I would recommend going to biddingfortravel.com for some tips on bidding. It was an excellent resource a couple of years ago, (I haven't been to the site recently).
I never had a problem with the hotel and/or hotel room. Some deals were much better than others. I enjoyed the anticipation of finding out whether my bid was accepted.
I'd go for it if you're not looking for a hotel on site - although some of the hotels on Hotel Plaza Blvd come up sometimes in Priceline. You may get lucky and get one of those.
Good luck!