Priceline doesn't have pricing power, they are just reaping the rewards of their branding and whatnot. There may also be some value for the consumer in that Priceline can consolidate their travel plans for flights/hotel/rental car in one place. However, I find I get better service when I transact directly with the vendor, whether it be hotels, airlines, or rental car agencies.
If you want the lowest prices on all the big hotel chains (Marriott, Hilton, IHG, Choice, Wyndham, Best Western, etc), then use www.roomkey.com I have no idea why they don't advertise it, but their loyalty pricing (you have to be a free member of their rewards system), and in exchange for the discount you get, the hotel doesn't have to pay commissions to Priceline/Expedia/other travel agents.