Gears of War is absolutely essential for all 360 owners.
Visuals: 10
The unreal engine 3 has allowed Epic to make a game with some truly incredible graphics. These are among the best you will ever see. Your enemies look very impressive, and the blood is among the best looking in any game I have ever played. The environments are often very dark, and always realistic. Even after playing more recent games, you will still be impressed by Gears.
Audio: 9.0
The sound of gunfire and footsteps sounds very realistic, and there is quality voice-acting by John DiMaggio and Lester Speight, among others plus a decent score. Just be prepared for the annoying screeches of the wretches.
Gameplay: 9.8
The gameplay in Gears is really cool and easy to get used to. It is cover-based 3rd person shooting with some interesting new mechanics. My favorite was the blindfiring mechanic where you can fire from behind cover without being exposed. There are also some seriously awesome weapons like the hammer of dawn and lancer (a chainsaw bayonet is a lot more practical than it sounds!)
Story: 8.5
On the planet of Sera, an alien race called the Locust has invaded, burrowing underground. You play as main character Marcus Fenix. The story was clearly not the main focus of this game, but it does get you wondering about Marcus' past.
Overall: 9.5
With smooth gameplay, great graphics, cool weapons, quality voice-acting, a semi-interesting story, and nonstop action, I'm not exaggerating when I say that Gears of War is absolutely essential for all 360 owners.