We used priceline in Disney for our 3/04 trip. We did save a lot of money. We did a lot of research first though so we knew when to stop bidding. We had a split stay. The first half of the trip we got the Sheraton Safari for $36 a night. This was a big savings as at the time 2 there were other diser's going the same timeframe's that paid the discounted rate of $99ish. I, for one, loved this hotel, especially for the price!!
The other wasn't a huge savings or a nice hotel, we got the Nikki Bird Holiday Inn for $26 a night. This place was a dump. On a brighter note I answered a survey when I got back home that Holiday Inn had sent me in regards to the hotel stay. I answered honestly and put in the comments at the end how disappointed w/ the hotel we were. I had never stayed at a bad Holiday Inn to that point. They proceeded to refund the 2 rooms I had won through priceline in full. I didn't ask or expect it, I was just completing their survey.
I would use priceline again for certain things. I just recommended it to a friend who is going to Baltimore for a long weekend w/ her DH.