As of December 15, Gears of War for the 360 sold an estimated 2 million copies. It has won several "game of the year" awards (including ours) as USA Today reports on its "break out" status. From the article: "It's no doubt [Gears of War] sold a lot of Xbox 360s and has made existing 360 owners feel very good about their investment. It came at exactly the right time and has proved to be a great counter to the launch of the PS3."
In the same article, Newsweek's N'Gai Croal disputes the game's "system seller" moniker but goes on to say, "Microsoft can hold up Gears of War and say, 'Sony says their machine is more powerful than ours, yet we have this game that looks better (than any PS3 games).' "
System seller? If not that then at least really good backup. Get it ... backup? (Cue lame sounding trumpet.)