All right, I'll stick up for Priceline. I've had great experiences everytime that I've used it, often paying less than half of what I found on other websites. Our favorite trick is to put in insanely cheap prices for a hotel in downtown Des Moines (where we live). Since most downtown hotels cater to the business traveler, they will take our bid for a weekend, and we get a cheap mini-vacation at a top notch hotel.
That being said, I have to admit that I agree with the others when it comes to WDW. You are investing enough money for other parts of the vacation that it would be horrible to let the unknown/uncontrollable ruin your trip. I have heard of people getting amazing deals on the Swan/Dolphin resorts, but there is no way of knowing if you will get those or another hotel, and once someone accepts your price, there is no backing out. Priceline has its place, and I will continue to use it for many of my hotel stays, but never for a trip to WDW.
I hope that helps. Feel free to PM me if you want to know any specifics about how I use it and how it has worked for me!