conqueror Kenny said:
NO! NO NO NO! I just beat that a couple days ago, did the entire campaign co-op with a friend. It's awful. Broken, imbalanced, and absolutely insipid. Don't waste your time. Besides, I don't think it's out for the PC at the moment.
If you're looking for a good co-op third-person shooter, I haven't played many on the PC, but I'd recommend Gears of War. The PC version has everything the 360 version has (as far as I know) plus the Brumak fight. Granted, Gears still isn't perfect, but it's light-years ahead of Ao2.
Other fun stuff is the Diablo games, quality RTSs like Rise of Legends, the Company of Heroes games, StarCraft, and WC3 (you can't do the campaigns together but doing online RTS battles and custom games with a friend is a LOT of fun).
Also, try the Tom Clancy shooters that have co-op (I think the GRAW and R6V games do, but don't quote me on that).