AO2 is a solid shooter. The co-op experience lives up to the hype. Oh and ignore the GS review, half of that crap is bs.
About the gameplay: The gamespot reviewer must have parkinsons or something, because i found it really easy to aim in AO2. Maybe he should just get good at shooters before complaining about not being able to hit anything.
The shooting is solid, the enemy ai, while not excellent is still good enough to get you immersed in the game.
The aggro system is cool, but yes, unrealistic. Why would the enemy completely ignore someone, simply because another guy has a better gun? but it still works cool and lets you do some cool flanking manouvers.
The co-op mechanics are cool, I especially liked going back to back, the game goes slow motion and you take out a wave of enemies coming from all sides. The co-op parachuting was fun to do with the sixaxis controls, but extremely hard for the person shooting while parachuting.
I also liked the fact that you can drag your hurt partner to a safe location before healing him. and he can still shoot at enemies while being dragged and healed.
This game is not bad like GameSpot would have you think. It is a good six to eight hour campaign, which has tons of replay value since you can play it with a friend. If you are still unsure about getting it, at least rent.