army of badass
Now, this is worth playing, if just for the badass-ness (if you will). I played through the single player, since my co-op buddy (you know who you are!) is too busy playing Soul Sucker (WoW).
The partner controls are easy to use and a snap to control aggression/cover/attack conventions. With co-op this would be even easier, as you don't need to control your partner (though you might want to at times).
Don't look for story. At all. This is just an excuse for two dudes to kick the crap out of about 30,000 generic terrorist-type people. And, for that, it's a lot of fun. And the game time is longer than expected for a co-op, 3rd-person shooter, so you can enjoy the carnage longer.
This is kind of the Doublemint of games – double your treasure, double your guns.
But, play it with a friend you like. One with a brain, preferably, or else you'll spend half your time trying to fight your way over to revive them.