Might not have reinvented the formula from the first, but Gears of War 2 is one hell of a ride.
Gameplay- Once again, you control your ragtag group of COG badasses. This time you take the fight to the Locust. Without giving anything away, I will say that the whole Geow plotline thickens considerably. There are a lot of new weapons, and now the arsenal is just awesome. You really have to try using the mortar and taking out a whole group of Locust, or using a flamethrower to destroy someone. It's awesome.
There are new moves too. If you wound a Locust, he will fall on the ground crawling in pain. You can use him as a meat shield to take damage for you, or you can use one of many unique executions on him as well.
The multiplayer is good. I wouldn't put it up there will COD4, but I was never a huge fan of Gears 1's mp either. Horde mode is fun though. Taking on wave after wave of enemies with friends is a great blast.
Graphics/Audio- Both shine. The graphics aren't a major overhaul from Geow 1, but they have introduced some color to the equation. The loals you go to a more varied as well. The lighting is done very well too. The sound is on par with Gears 1. It's not a step up, but that wasn't to be expeted from me at least. It does it's job, and it does it very well.
All in all, if liked Gears of War 1, Gears 2 will be more of the same, but you know that's a good thing, right?