Technically speaking, Army of Two is solid and sometimes outstanding. You may get a little tired of the over-egged HDR effects, but the Unreal Engine 3 graphics pack in plenty of detail, particularly in close-up shots of your brutishly armoured duo. Audio is on the gritty and understated side - though that's no bad thing in a game soundtracked by the constant rattle of machine-gun fire.
The game also feels robust on the networking side. Occasionally the action will pause a little while the game waits for your partner's console to catch up, but generally speaking we're talking about a smooth, lag-free experience. The only shame is that you can't kick in a single player game and have partners drop in and drop out at will. Instead, you have to select an online co-op game at the opening (starting at any of the missions you've unlocked) and go from there.
I won't go overboard dissecting Army of Two any further. It's a strong co-op focused title and if you've enjoyed similar options in Gears of War or Halo 3 then you'll certainly get a kick out of this. If you're in it for single-player, however, then there are better ways to spend your money and your time.
My only real worry is that it's not a huge game, and while the game also packs in an interesting competitive online mode, still based on the same two-man, co-op concepts, it won't give you as much long-term value as Gears of War, Halo 3, Call of Duty 4 or Frontlines: Fuel of War. All the same, it's a good jumping off point for future co-op ventures, and if that means another Army of Two with a little more spice to the levels, then I for one wouldn't be too disappointed.
An innovative and well-designed co-op shooter that, despite a decent AI, only takes off when played with another human - preferably a real-life buddy.