After completing this on the medium difficulty setting with a friend within one day (10hrs), I would say that Army of Two is a great co-op game, mixing Gears of War and Ghost Recon Advanced Warfighter together. The hand-to-hand is a bit dodgy, but apart from that the fighting is decent, producing intense long and close range combat. There is a good varity of weapons, each being customisable as well as having their own unique feel. I didn't play the game online, nor without a friend taking the other characters role, so I have yet to test the AI of the partner. However the enemy AI is effective, with it flanking and using decent, not mindless, tactics. The game is quite un-pc in parts, making you shoot suicide bombers that run towards you like lemmings, but it flows well with the games tongue-in-cheek humour. My only critisicm with this game is the fact that it is so horrifically short. This is simply due to the fact that as it's a multiplatform game, it has to fit onto a DVD for the 360, and yet has to have good graphics. This leaves the game a maximum of 12 hours to complete it 100%. The 8gb limit of the DVD really hinders many cross-platform games such as Devil May Cry 4, the Tom Clancy games and more then likely GTA IV.
In summary, Army of Two is great to rent out for a night with a friend, but not worth paying more than £30 for due to it's length.