Picked it up and played like 5 hours of campaign today. :lol It is simultaneously frustrating (some cheap sequences) and jaw-dropingly awesome (everything else).
The water and (yuck)
parts are pretty stunning technically and design wise, everything is so intense and massive in scale. Gears set pieces squared. Where Gears 1 was small-squad tactics, Gears 2 is (or feels like) larger force clashes.
The AI is massively improved, you can actually use them as line holders and keep back with the sniper rifle and pick people off. In the first game they were practically not there.
And the story is much more fleshed out and involving, I actually hate the locust, which makes killing them in ridiculously gory moves even more satisfying. Though I bet many are going to go through thinking "wtf is this story in my gears bloodbath?"