The Upgrade Travel blog has an excellent look up at various online fare aggregators, trying to find the best one to do your online fare searching with. He compared Kayak, Sidestep, Farechase, Mobissimo, Pricegrabber, Berzurk and Farechaser for accuracy, depth of information and output control.
The result was that Kayak came out on top, way on top. The prices were lower, the user could control output with greater efficiency and ease than other aggregators and breadth of information.
I’ve personally stopped using fare aggregators, although this likely has a lot to do that options flying from Ireland to Europe and the US are limited unless you’re willing to fly to London first. But if you’re looking for the cheapest way to join the Chicago to Dallas branch of the Mile High Club, Kayak’s the way to go.
Disaggregating Fare Aggregators [Upgrade Travel]