I 'unno. Halo does kick ass, and the two games that they've got in the pipeline sound pretty badass, but the mutliplayer of 1, 2 and 3 kinda sucked, imo.
Cod4, Battlefield, Dark Messiah of Might and Magic, STALKER: Clear Sky, Dawn of War and ET: Quake Wars seem to be fulfilling my multiplayer needs, right now.