I use priceline for hotels when I'm fairly certain of the hotel I'll end up with. Bidding in an area on a star level where there is only one or two hotels for example pretty much eliminates any chance of surprise. You don't have any choice of room usually, so you risk ending up with a smoking room for example if you get a hotel with them, but that's how it goes.

For example, if you bid 3.5 stars at Laguardia, you will probably end up with the Crown Plaza or the Marriott, for way less then half of the going rate.

I don't use them for airfare, I prefer to have way more control over my airfare.

Until their rates went way too high, I also had very good luck with them for rentals, usually ending up with Hertz or Avis.

There's always a risk it will be something you don't want, but you are paying hopefully a fraction of what it would normally cost, so you accept the risk for the savings.