I don't use it because, last I checked, it's unable to handle what I think of as basic corner cases, like booking a roundtrip going out from one airport in a metro area and coming back to a different one.
E.g., depart Chicago from O'Hare to Denver, then return to Chicago from Denver, arriving at Midway?
Choose Multi-city, and specify specific airport codes at the originating / terminating metro area. E.g., American 1017 ORD to DEN on 7 Jan and Frontier 534 DEN to MDW on 14 Jan for $211.
Replying to myself since I can't edit: I see two replies suggesting that what I want is a multi-city trip, but that's not the same thing. Flights will happily let me do the same search I can do on Orbitz, with my start/end city being "SF metro area, any airport". But unlike Orbitz, when I use that I can't book an itinerary that goes SFO-BOS-SJC, for example. (I don't want to search it as multi-city, because then I have to _manually_ try every combination of SFO/SJC/OAK until I find what's cheapest.)
