I almost always use Priceline for booking rental cars, cause it saves money, and who really cars that much between Thrifty, Avis, Hertz, Enterprise etc.
A couple of times for hotels, it was ok but nothing too earthshattering.
On the other hand, I find it somewhat risky to use it for booking airline flights. You run the risk of being locked into some horrible flight schedule, inflexible travel arrangements, and for me the kicker is no frequent flyer miles which for me balances out the cost savings.
On the other hand if was very confident of my schedule, either way in the future or you need to catch a flight in the next day or two... you can save money with priceline.
I am curious has anybody else being finding the best deal on flights is use Orbitz etc for finding the flights and then booking directly with the airlines, or is there a new cheap airfare travel site which is consistently a good source of bargains?