Co-op at its best, but also a functional AI is here.
In Army of Two you play as a mercenary in a 2 man team working for a private military corporation. These men fight in Somalia, Afghanistan, Iraq, China and Miami through a short but ultimately rewarding and different story line: this time you are a simple mercenary instead of a noble freedom fighter or something. Story starts of strong, but in the end boils down to a very typical and predictable conspiracy plot. But it's the different mercenary setting of the game that drives you forward and not the story itself.
Graphics are good, especially in cutscenes and character models, but for some reason the environments look a little too grainy. Color palette is mostly brown, which is due to the fact that most of the game is played in a desert environment.
Music is orchestrated and somewhat different while still bringing memories from Gears of War. Sound effects on the other hand could have used some improvements and most of all: the sound of firing a gun is terrible! There's no masculine feel or kick at all in it. Voice acting and dialog works well and there are some memorable one-liners throughout the game.
When starting to play I strongly recommend that you forget about Gears of War: Army of Two doesn't play like that. There's no specific covering system per se and it takes a while to get used to the different controls. For me and my friend it took about a 3 hour worth of gameplay to really get into the game.
Army of Two relies on 2 different gameplay tactics: cover and Aggro. This means that your partner starts to shoot behind cover growing his Aggro Meter and in this way drawing the enemies' attention to himself. In the meantime you can run behind the enemies and kill them very easily. This is used throughout the game and even though it gets predictable it's always rewarding to do so. For some reason you can't shoot through any object excluding a glass window: even an American flag is bullet proof. If there's ever a sequel I really hope that this mistake is fixed.
Every time you complete objectives in the story mode you gain money which can be used to buy and upgrade new weapons and also to buy new masks to wear. You get to keep all your weapons and gear after you unlock them once so playing the game gets always a little easier when doing it again on a higher difficulty setting for example. There are 2 difficulty settings to choose from: Recruit and Contractor, but after finishing the game once you unlock a harder Professional mode. All of these things combined will make you want to play the game multiple times with different friends and using different difficulty settings.
Save system uses checkpoints and only checkpoints which in a few occasions are way too far between one another. Fortunately this happen only 3 or 4 times in the 6 or 7 hour experience.
Enemies can really be a pain in the ass. Their movement is usually so unpredictable that it's hard to hit a moving target. AI is also good so they always use cover and even know how to change their position while staying behind cover. After dying they seem to lose their mass and stamina just like in Gears of War and this is always a weird thing to notice.
There's of course also versus multiplayer and all of the game modes are somewhat objective based. This is a good thing, because matches are only 2 vs. 2. This number is extended by putting additional enemies a.k.a bots in the game. There's some fun to be had here, but I don't think you will be enjoying versus multiplayer for very long. If you're specifically looking for an online shooter I suggest you play Call of Duty 4 instead.
There's also 2 noticeable flaws when playing multiplayer: matches are too seldom played from start to finish, because if 1 of the 4 players leave in mid-match it's game over for everybody and you need to start all over again from the main menu of the game. You can also play online only with player whose game is the same region as yours. This is very unusual choice and a weird thing for EA to do.
Ultimately Army of Two really is a great game despite its flaws and shortcomings. You probably want to play the game through multiple times and the online multiplayer is worth checking out. "Now go out there and kick some ass!"