I used priceline alot too

Check hotwire first to see what is the going rate of 4 star hotel. After that, bid on priceline. Remember, starts low.

I have done it 4 times recently with very good results, getting Hyatt, Westin, Sheraton etc for $50 - 60 per/night.

Yes, there are fees which is about 20%. So, when you bid $60, the final cost could be around $72. Also, you need to have some understanding of other costs. For example, all hotel at SFO charges at least $18 for one night parking.