I've used them several times. There are a couple of things that you have to do and understand in order to get a good experience.
#1 Read betterbidding.com and biddingfortravel.com extensively. Have a very good understanding of what they mean when they say Free Rebids, what the zones are, what hotels consistently participate, and what the current winning bids are running.
#2. Understand that Priceline guarantees you a room in the hotel with a bed. Most hotels will try to accomodate requests such as two beds, non-smoking, a crib, connecting rooms or whatever -- but they are not required to and if the hotel is full you can bet that you will be fit in around all the higher paying customers.
#3. Priceline takes a bit of adventure. The lists on the websites are who has won in the past and what hotels they have won at. New hotels can be added all the time, hotels that were previously 2 stars all the sudden win at three star categories etc.
#4. My own personal rule -- I don't bid on anything less than a 3 star (which really are more of 2 star hotels) and really try to only bid on 4 star or resorts. In all the places I've stayed, there are some really scary and/or yucky places on those 2 star lists.
#5. If you mess it up -- NON-REFUNDABLE. You will live with it.