This game combines a shell of a good average shooter with the fun great coop. Grab a friend and enjoy!
The game is built around a borrowed concept from many MMORPGs: Aggro. And while many WoW fans will catch onto this concept immediately, others it may take a bit of explaining, but either way you will catch on quite fast. It is best explained by the drill sergeant in the opening cutscene: if you gun and you fire at enemies, they will back at you, allowing your friend to do whatever he wants.
The single player campaign is good, but it has its flaws. Your AI partner wont always perform the way that you want him to, sometimes holding a position outside of the cover you want him to stay in, or "sneaking around" right in front of a heavily armored enemy you need to flank, mostly due to the lack of a precise "go here button" (think Mass Effect's LB) that is instead replaced by a three choice movement pattern on the D-pad. You can tell your partner to either stay put, advance, or follow you and do so either aggressively to take aggro or passively while you cover his back taking aggro yourself. At times your partner will make bad choices, choosing to drag you in the opposite direction of cover to heal you after you have been downed, or even refusing to fire in a crucial "coop snipe" maneuver. Flaws aside, your AI will mostly do what you want if you tell him correctly and cover him well, and you will find a good amount of fun to be had even alone.
The game truly shines in online or split-screen coop, where any AI problems are replaced by a friend or someone who decides to join you. In this mode you can truly master the aggro system and manipulate it to your advantage, allowing a buddy to snipe at enemies while you blind fire safely from cover, or launching a grenade or rocket at an armored enemy while he is distracted. This really allows you to play the game to the fullest, and it makes all the difference in the long run; replacing frustration with satisfaction at pulling off something difficult. But all in all there is no escaping that the story mode is generally short (I spent about 5-6 hours on my first playthrough with a friend on Recruit difficulty) but the achievements and rewards, as well as an opportunity to further customize your weapons, by playing through multiple times on higher difficulties will mean you will see Rios and Salem through their story more than once.
The story itself is surprisingly interesting, I went in suspecting not much more than two guys mowing through hundreds of people, and while thats just what I got it was packaged with a gripping storyline that was surprisingly relevant to today's Army and PMC events. The game was even unafraid to show images of the Twin Towers going down to add to the storyline, and the characters even seem to rag on the military before they delve deeper into the storyline.
Multiplayer is nothing revolutionary, you won't find your typical shooter deathmatches here though, only 2 vs 2 objective games. But with a friend it can be quite fun as well, just nothing new.
Overall, buy this game for the coop, rent it for the single player and multiplayer.