This game starts off the Gears of War trilogy brilliantly. It is dark, grimy, and it feels heavy.
Your enemies in the game are called the Locust. They are humanoid creatures that live underground, and burst up through Emergence Holes, and try to gun you down. Unfortunately, the game doesn't go into too much detail about who, or what, the Locust are, or even why the are attacking you. But, you spend so much time trying to fend them off, and complete objectives, so this lack of information isn't that noticeable.
You play as an elite soldier, with a big gun. What Gears does differently here is that it gives you a chainsaw bayonet. This doesn't seem like that big of a deal, but it does become an integral part of the game (in terms of both gameplay and sales), and it is a whole lot of fun to reduce your enemy to an arm or two and a whole heap of fleshy chunks of meat.
The game itself is a basic, linear shooter. In a nutshell, the game is basically stay alive, kill lots of Locust, and get a bomb deep into the underground to destroy the Locust tunnels and end the war.
It supports two player co-op, and up to eight people in an online multiplayer match.
To sum up, if you want a nice, dark, heavy shooter with a fairly lengthy campaign, then this is a definite must buy.