So-far, it seems to be alright. I've only done the first level so-far, and am on the second (my Xbox freezes after 15 - 30 mins of playtime, so in-order to play, I have to keep turning it off and on - which gets rather annoying).
I love the customization so-far, that seems to be a very big, and great part of the game. As-for the gameplay, I honestly don't see this as being anything close to Gears at all. I like the gameplay so-far though, but I haven't experienced anything really "great" yet.
I'm sure it's also because I'm starting on recruit, but it seems as-if I really don't need my partner at all to get through the levels. He's very helpful in some spots, but it's almost as-if I could just beat it all w/o him.
As-far as AI goes, besides one time where my partner was just running around in circles, I haven't had a problem with it yet, and think it's actually rather good A.I., so I don't see why anyone really has a problem with it, the A.I. seems to flow pretty good according to the situation at-hand, not amazingly, but still seems to be very fluid.
I'm on the fence about buying it though, right now I have it as a rental from GameFly, and was going to instant-buy it if I thought it was good, but right now, I'm still not sure whether or not it'll be worth it.. then again, I have yet to play co-op, which I'm sure will 100% change my stance on the game and make it an instant-buy.