Army of Two is possibly the best co-op experience available. Buy this game if you have someone to play with.
Few games have ever had such a deep co-op experience as Army of Two does. Previous attempts have amounted to two players playing with each other, but at the same time by themselves. In Army of Two you can't play with someone and run off in a separate direction or completely forget about your partner. There is a built in reliance on your fellow soldier, a sense of companionship that can't be had in any other game. You'll finish this game knowing you did it as a team, and you'll think it was awesome.
If you strip this game down to its bare bones, its a shooter. You shoot to kill, you go towards objectives, you find cover to regain health, but then theres a few more things thrown in. You get the aggro system, upgradeable weapons and armor, a saving system for when one of you goes down, secondary objectives, unlockables, and on top of that you even have competitive multiplayer.
The extra mechanics work very well. The aggro system mentioned above is a great strategy to use as well as a way to know when to find cover and when to become more aggressive. You could have one team member get the enemy's attention, while the other fills them full of holes from behind. This is also an essential tool at some points, as some enemies can only be hurt from behind. A very cool bonus for using the aggro system is also incorporated. If one person is at full aggro for a certain period of time, you enter a special mode where the one currently attracting attention moves slowly but deals double damage, and the other becomes invisible and can use melee attacks on enemies, however, this bonus only lasts a short period of time.
Say someone is under heavy fire and is at the brink of death, he can use a "feign death" system, falling to the ground and transferring all aggro to their teammate. This can be a life saving system, or a fun way to really piss off whoever you happen to be playing with. If you do go down though, you enter into a system much like that of Gears of War. Your partner must get to you and bring you back to life. Your teammate can drag you to cover and then shove a tampon into your wounds.
If you're playing with a computer, you can either show affection or disapproval to them. By walking up to them and pressing the A or R buttons, you can either pat them on the back or hit them. This can tell the computer that what they just did was bad or good. If you piss off your Partner AI, he can do something like refuse to cover you or even hit you back. The PAI is about as intelligent as you would expect it to be, but you can issue it commands via a radial in the bottom right corner of your screen, which is also used for weapon selection.
The weapon system is executed extremely well. You can buy new guns with money collected from completing objectives. These arms fall into three categories - Primary, Secondary, and Special. The Primary contains shotguns, assault rifles, and Light Machine Guns. The Secondary has pistols and some Small Machine Guns. The Special section contains the "Fun" weapons such as snipers and nasty rocket launchers. You can buy accessories for most of these guns such as stocks, barrels, shields, new cartridges for ammo, etc. You can even pimp out your guns, coating them in gold, diamonds, and making them shine. If you're feeling especially loose with your hard earned cash, you can buy new paint schemes for your masks. These range from flames to clown paint.
The campaign is surprisingly short. This is a let down, but at the end of your first play through you wind up completely ready to play through it again on a harder difficulty in order to get new guns and whatever else you want to buy. The narrative itself is one of betrayal and military conflict. The actual missions themselves take a backseat to the political side of the story. A bill to privatize the army is trying to be pushed through, which would replace the entire army with private contractors like Rios and Salem. There is not much of an emotional attachment to the characters, but at the end of it all you still wind up caring about them. Then again, would you want to be emotionally attached to mercenaries toting serious firepower?
The competitive multiplayer is sort of a shrunken down version of the campaign mechanics. You buy weapons and weapon packages in between rounds as you and a partner fight another team of two. This is surprisingly fun, as it works just like it should. You win by using tactics such as the aggro system, although the meter is not on the screen, of course.
If you don't have someone to play though the campaign with, rent the game before you consider buying it. This is without a doubt the best co-op ever made, and I can only hope for a sequel.