Demo video that was posted to YouTube.. skip forward to 3:45 for gameplay.
As much as Gears of War has been overhyped, I'm pretty impressed with the battle mechanics. I like the speed which the player moves from cover to cover and slams in there. I agree with one of the commenters on YouTube that the AI looks pretty lame. Did any of the bad guys get shot by the AIs in this demo? I hope that the real game has smarter cooperative AI than this. One of GRAW's (Ghost Recon: Advanced Warfighter) major failings is that the AIs in cooperative multiplayer online are horrible. My buddy and I just picked them off as they came up a hill one after another, that was the one and only time I played GRAW in co-op.
As far as AIs go, I have to imagine that Army of Two has to have a superior AI if it is to be any fun. Here's an Army of Two trailer that's all prerendered and tells us nothing about the game except that there are two soldiers.