The only problem is that, while Army of Two functions as a single-player game, it's not actually all that enjoyable as one. After the novelty of the first couple of levels has worn off you feel like you're just travelling from one cave, camp or concrete bunker to the next, fighting off more of the same identikit middle-eastern terrorists using basically the same technique. Instead of Halo's infamous thirty seconds of fun, over and over again, we get thirty seconds of time passing in a vaguely interesting fashion, repeated ad nauseum.
Tactics never seem to develop and there's just not enough variety to the environments or the challenges posed. The game picks up with an assault on a terrorist occupied aircraft carrier, but it's not long before even this gets bogged down in more of the same-old, same-old. As a single-player experience, I would comfortably put Call of Duty 4, Gears of War, GRAW2 and even Frontlines: Fuel of War way ahead of Army of Two. It's full of interesting ideas - I'd love to see the weapons store and upgrade options turn up in other 3D shooters - but it's ironically not all that exciting.
With a friend, however, it's a different story. Even played with a stranger over Xbox Live, Army of Two takes on a new dimension because the simple fact that you're playing with another human being adds a range of new dynamics to the game. You have to fight well and look after your partner, partly out of embarrassment at what happens when you don't, and partly because you need them to do the same for you. Even in the space of a single mission a sort of trust develops, and you learn to work together as a team, taking turns to cover, suppress and outflank so that neither of you is left out in the fire for two long.
You learn to take calculated risks, not stupid ones, because collapsing in the open under heavy fire will only get both of you killed. In a game where checkpoints can be quite spaced out, this is never the most appealing option. If your partner has a headset you'll soon get an idea of how pleased or pissed off they are, and even if they don't a system of single-button applauds and rebukes will give you an idea.