You can usually get pretty good airfare from Priceline, however, you have to be very flexible with your travel plans. Unfortunately you don't know your itinerary until after they've accepted your bid.
Like Serene stated above, checking out Yahoo travel, Orbitz, Expedia, etc. is a good idea. Prices are usually comparable to eachother. I have to say I've had the most success with Yahoo Travel and Orbitz. Also double check with the airline directly...this usually does not work for me, I usually find the better rates on the other sites.