I've had great experiences getting hotels rooms at significantly reduced prices on both Priceline and Hotwire -- my best deal was more than 55 percent off a room in New York. While both services allow you to know or designate a geographic area, star-rating, and price (Priceline actually allows you to bid), neither site lets you know exactly which hotel you're getting until your offer is accepted.
Frankly, I think a good part of the reason I've never had much of a problem is because when I've used these services I tended to stick to 3-star and above properties --- my own personal experience is that when you go below that rating the range of cleanliness and quality can vary greatly so I want specifics before I go 2-star.
But for those of you made squeamish by the potential risk, Sean O'Neill at the Budget Travel blog offers some tips on how to game Hotwire's system. Hotwire offers prospective buyers more information in hotel amenities than rival Priceline. O'Neill says that he's had great success plugging that information in on sites like Bid Goggles, Hotwire Revealed, Bidding for Travel, and Better Bidding, which are designed for just this purpose, to suss out the likeliest suspects. He notes that the system is not completely foolproof, but seems to work pretty well and could go a long way toward allaying anxieties over the mystery element of scoring a cheaper room using one of these sites.