Short but sweet
The original Army of Two was one of those games that sold a lot of copies yet no one seemed to care much for it. 40th Day is in the unfortunate position of being a sequel no one asked for. Luckily it seems that most of the criticisms leveled at the first game were taken to heart in designing the sequel.
The story is paper-thin. Essentially a pair of mercenaries, Salem and RIos, happen to be in Shanghai when a whole bunch of bad shit goes down. The story in these games is never more than excuse to get to the action, and it's nice that they don't bog down the pace with too much exposition. Story-wise the main hook seems to be morality decisions. You are given a pretty black-and-white choice every mission. Basically the extent to which these actually effect the story is pretty limited though. It essentially boils down to extra weapons or extra money. It would have been nice to see it unfold in the story somehow.
Gameplay is based around an aggro-meter. One player is tasked with drawing enemy fire and attention while the other stealthily flanks and quietly dispatches of his foes. In single player you control your AI partner via commands (hold position, cover, push forward), it's basic but it generally works well enough. I found that the number of bone-headed AI decisions was on par with the times he ended up saving my ass.
The controls take a little bit of a learning curve to get used to. Think Gears of War just not quite as smooth. Once you get used to the cover mechanic the gameplay starts to feel pretty solid. There is a neat weapon customization tool that allows you to spec your weapons with larger ammo drums, different muzzles, camo styles, as well as being more or less aggressive. I found the system to have enough variety and depth to be worthwhile without being too technical.
40th Day is a short ride so it's smart that it has a pretty robust multiplayer suite. CO-OP is easy to jump into and I had no connection troubles whatsoever. This is definitely a game where you will want to have good communication going with whoever you're playing with. It also features some Gears-esque more traditional deathmatch/wingman options which may or may not have legs beyond a short launch window.
If you can appreciate a well rounded, but basic co-op shooter then I think your $60 would be easily justified. It looks pretty, and it plays well both online and off. Innovation was not the intent, so if you want something that pushes a genre you may want to look elsewhere. Everyone else should just enjoy the ride.