Gears of War had one of the most phenominal single player games in the history of gaming (and I've played every FPS/TPS on the 360 up till the last month).
What made it one of the best SP's? Online and Offline Co-Op. Can't find a friend? Plenty of people and incentives to beat it on co-op either way (aside from such a great experience).
IMO, you can complain about SP's being gimped due to MP, but if anything, MP co-op campaigns against AI is just about the funnest experience out there, provided you got a good group of buddies to play with.
Back from the dead, I'm afraid.