I don't have it yet, but I had the chance to play the co-op campaign for several hours on monday round at a mates. It's very good - there's no drastic changes in particular from the existing formula but it feels a little more dynamic and well paced - like the pacing between sections and all-out firefights seems better, and kind of makes the heavy action bits more impactful. I didn't play through to the end but I think it's longer than GoW1. There's some nice new weapons inc. flamethrowers and the graphics look as excellent as they did first time (unless you dislike the art direction/style). Some of the outdoor areas and underground sections look particularly nice.
I'd probably say it's more of the same but a bit bigger and better than before. I wasn't expecting too much from it, but found it very enjoyable. I didn't play the multiplayer, as I've never been a fan of it online, so I can't really comment on how it plays or what new additions have been added. I'm sure some of the GoW addicts here can probably shed some light.