Tanshin and I got a good 5-7 hours in on Ao2 over the last two nights, and we've had a ton of fun after a rough start.
The tutorial/intro level to the game is pretty poorly explained, we had a hard time getting a hang of some of the actions like sliding into cover, and a lot of the sections of the tutorial have completely unclear objectives so we'd end up just standing around shooting each other for a bit till we figured it out.
Most of the control problems stem from the A button being the "Context action" button, like in Gears of War and Zelda. However its not as well implemented in Ao2, so you end up doing some unintended actions (most commonly, feigning death when you want to activate your "overkill" power.)
Once you get into the thick of it the game really shines though. Every segment is built around co-op play so you're constantly providing cover fire for your buddy, dragging him to safety (as you both shoot!) while he's wounded, setting up pincer traps for enemies, etc. One of the cooler features is the "aggro" system, liberally stolen from MMORPGs. Basically the player dealing the most damage gains the focus of all the enemies, while the other player can then sneak unnoticed behind the front and flank the enemy or complete an objective. It adds a nice layer of strategy to the standard run-n-gun experience. You also have a constant Picture-in-picture of the 2nd players point of view, so you are never wondering "what the hell is the other guy doing right now?", and there are simple commands you can make your guy yell like "Move forward!" or "Come over here!" in case you don't have a talkative partner.
You can interact with the other player in a positive or negative fashion, so like if they screw up and get you killed you can headbutt him or slap him in the back of the head, and you can also do
RESPEK KNUCKLES and other congratulatory actions if you feel like you did something cool. Or, congratulatory headbutt as is Tanshin's custom.
You can also swap weapons with the other player mid-battle, in case a change of tactics is needed. One (seemingly) cool thing that we haven't really gotten the hang of yet is the simultaneous snipe. You can "call it" out to the other player, and when he accepts you go into this split screen thing where you can see what the other player sees and initiate a countdown to where you both snipe different targets at the same time. Good for taking our guards or key targets without alerting everyone.
There's also a few cool segments each level where you do team-up moves like going back-to-back Mr. & Mrs. Smith style to fight off big hordes, tandem sky dives, and two-man hovercraft combat.
The most addicting part of the game is probably the weapon customization. Being a PMC contractor, you earn money for completing your primary and optional mission objectives, and you can use that buy new weapons and customize them. Every weapon has a lot of options to change, like barrel type, attachments, stock, etc, which change the stats of the weapon to your personal playing style. You can also "pimp out" the weapon with gold trim, engravings, etc. This serves no purpose except to infuriate the enemy with your flamboyant audaciousness, so it provided a big boost to the aggro generated by the weapon
The story is pretty predictable action-movie fare with a touch of "ripped-from-the-headlines" sensationalism, but the over the top cut scenes and intense in game battles keep it moving at a brisk pace. All in all, so far the game is fun as hell, and I'm chomping at the bit all day at work so I can get home and start teaming up again. Hopefully this is just the beginning a strong resurgence in the co-op style of gameplay.