The 40th Day is your regular third person shooter, with a number of other things that make it stand out from the crowd.

User Rating: 8 | Army of Two: The 40th Day PS3
I've never played the original Army of Two, but this is meant to be quite a significant improvement.
The 40th Day is an extremely solid third person shooter. It has your standard action, but this is mixed in with co-operative two player based gameplay, a huge number of customization possibilities, and overall it is just above average, although there are some flaws.
I'll try and explain the story... Our two main mercenary protagonists Rios and Salem (Salem being voiced by Nolan North) go to Shanghai to complete a standard job and earn a little cash.
Then Shanhai blows up, and the pair are caught in the middle of it.
Why is it blowing up? No idea, but they are caught in the middle of it. And that's pretty much all that matters. I tried to follow the story, but The 40th Day does a poor job of filling you in, and really, it doesn't need to.
Even though all the voice acting is quite good, the story matters zero.
In the vein of huge summer blockbusters, Army of Two: The 40th Day is all about the action, in which it does an extremely solid job, even if we don't know why it is called The 40th Day.
The core shooting gameplay is cover based, sliding into cover is automatic and easy. Since going into cover just really happens without the press of a button, it's suprising that it doesn't get in the way, with over being generally all over the place.
There is some awkwardness with the movement, but it's not game breaking in anyway and doesn't really cause any major problems.
The X button is used for quite a few things, rolling, sprinting, opening things, co-operating with your partner, and occasionally sliding into cover from a distance. Again, this works suprisingly well.
If you've ever played Uncharted, the shooting is quite similar to that game, although a little less refined, it holds up well, and is extremely fun.
Army of Two is unique because of the co-operative gameplay which can be played solo with a computer controlled ally, split screen, or online with a friend or stranger.
Since I usually play my games offline, and I don't own a second controller, I finished this in solo.
I played a bit of the game in co-op which is obviously the way this game is meant to be played, and by all means, if you're going to have a mate over for the night, and want some mindless, two player game, go and rent Army of Two: The 40th Day, providing your partner knows what their doing you'll have an absolute blast.
And chances are, you'll finish it together. Army of Two is extremely short, which is one downfall of the game, so luckily it has online multiplayer to back it up. But I'm getting sidetracked, more on that later.
The split screen works well, but chances are you'll spend most of your time playing this solo.
And this is also works extremely well. Challenging, but in a good way. On the default difficulty, The 40th Day puts up a fair fight.
I found myself to usually just run in shooting and not worrying about my co-op companion too much, which usually worked, but the game makes sure that you do need to watch eachothers back, solo or not.
There are some interesting mechanics here, my favorite being the mock surrender.
You can walk in to bad guy territory and pretend to surrender, and have your partner flank around and take out the baddies from behind.
Or you can just quickly draw your gun and take them out wild west style.
Another great mechanic is saving hostages.
You never have to save them, but if you do they often help out.
Saving them can actually be quite difficult at times.
Basically you have to capture and hold the commander at gunpoint, and the other bad dudes will surrender, in which you can order your partner to tie them down or execute them. Or both.
There's also morality choices, which are interesting since you are always shown the weird and often bizzare consequences of your choice in stylish comic book sequences.
The system could have been deeper, but it works.
Customization is also a huge part of Army of Two. There are a huge variety of ways to customize your weapons, and the system is simple, but addictive and fun.
All the weapons are easy and fun to shoot.
There's also competitive online multiplayer which I didn't actually play, but that is also meant to be quite solid, also implenting some co-op tactics.
So there you have it, Army of Two: The 40th Day is a great game and a solid third person shooter, which is unique enough to have its own feel and flavor.
It is quite short, so maybe just rent it, but if you enjoy the online multiplayer then The 40th Day is definitely worth your time and money.