I love Priceline and use it all the time for hotels and cars, but never for flights because of the uncertainty of the flight schedule that you will get. Just last week we got a Marriott Renaissance Sport hotel in Aliso Viejo (Orange County CA) for Xmas week at $60/night.
In mid-size and larger cities, we usually get rates of $35 - $60/night at national chain hotels like Red Lion, Hilton, etc. especially if we are trying to get a room on the weekend. (Business hotels are usually much more available on the weekends). Our best scores have been in London where we've always been able to get rooms in typical tourist class highrise hotels for $100/night + tax in Kensington near Hyde Park and the Natural History Museum.
For cruises, I think it would work as long as you aren't picky about your exact route or the cruise line. If you have a specific route or cruise company in mind, you won't want to take a chance on getting something different. Also, the key to the Priceline "name your own price" option is to do your research beforehand so that you know what is available as a "normal" good deal, then bid at least 30-40% below that and see what happens.