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.
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.