[pinko]

It's not just about prices, it's about routings. For roughly the same price (give or take $100), I often have 50-100 options for each segment, of wildly varying convenience and robustness (in terms of flight time, layover, delay likelihood, airline, etc.). Their system does an incredibly good job laying out this info in a comprehensible form, and more importantly, providing an advanced routing language that allows me to say, for example:

MSN :: AA ORD AA X AZ

BLQ :: AZ X AA ORD AA

...which means I want an American Airlines flight from Madison, WI, connecting through O'Hare, and continuing on to Bologna, Italy, with one and only one intermediate connection, with the over-the-pond leg on AA, and the final leg on Alitalia.

It can then lay out the options for me graphically, so I can instantly see which flights have nasty layovers or bad schedules without having to parse a single text digit.

It's not just about price.