The 40th Day is decent, but it could be better in many ways.
Firstly, lets get this out of the way. On the PS3, this game's framerate is horrible. I would recommend purchasing the Xbox 360 version, becuase EA have some sort of problem with keeping a consistent frame rate on their PS3 games. Other games like Skate and Medal of Honor: Airborne have these problems. Why can't they fix this? Whenever you enter a large area in Army of Two, all the nice looking graphics just look ugly becuase of the frame rate...
Ok, onto the actual game. The game is basically set in Shanghai, and you play as either Rios or Salem, these 2 big dudes with guns who get caught up in a terrorist attack, which literally decimates most of the the city. The graphics look pretty good, but the explosions look kinda bad. The graphics are mostly average, apart from the environments, which look pretty nice, similar to the Uncharted 2 level of quality. The game is a 3rd person cover based shooter, and all the mechanics work well enough. I did find the controls to be a bit daunting at the start, but afterwards I got used to it.
The game offers a large amount of customization, which I found to be downright awesome. The amount of stuff you can do to your guns is amazing. You can use a soda can as a silencer. You can paint your guns gold to annoy the enemy. All these different things come together into a really awesome weapon customization mode that really has a lot to it. You can also customize the masks that Rios and Salem wear. There are some presets, but in order to create a mask yourself, you have to log onto EA's specific Army of Two website, and that complicates things. Why not just include an editor inside the actual game?
So, overall, Army of Two: The 40th Day is good in some ways, but bad in others (god-damn framerate! Argh!). So yeah, if you enjoy cover based shooters, give this one a try. But, if you aren't, I really can't recommend purchasing this game.