We're using AToW for our 3SW game, but I do make extensive use of older, first edition books. We're using the fluff, monetary values, 'Mech rarities and other background info from MechWarrior 1st Edition, The Mercenary's Handbook, JumpShips and DropShips, the original TRO: 3025 and 3026.
Since we're playing in 3SW, we're using a LOT of material about nobles, landholds, etc. Our unit is made of the scions and retainers of the Houses Minor that rule various parts of the planet Royal. Our initial decision to use AToW was based on the desire to use the latest-greatest set of official rules - a decision that has been rendered moot by the interminable wait for Interstellar Operations. We've cobbled together our own resource management for landholds by adapting information from A Song of Ice and Fire.
Without a current rules set for managing a command, it seems like we're using about 33% AToW and the rest sources from older Battletech books and other RPGs. It'd be awfully nice if Interstellar Ops were out sometime relatively soon...