From the review on Metacritic’s site:

Gears of War 2 on Metacritic

Gears of War 2 builds upon the gameplay of the first title in every way. First, the scope of the action is bigger: new characters, more weapons, nastier enemies, bigger vehicles, and grander locales offer more action, more emotion, and more thrills. Next, every major aspect of the game has been refined to create heightened, more immersive experiences. Finally, the overall tone of Gears of War 2 is more badass: replete with new weapon-specific executions, chainsaw duels, and shocking boss fights. Gears of War 2 raises the graphics bar for next-generation games through its breathtaking visual effects, which are made possible through advances in Unreal Engine 3. New visual effects include ambient occlusion, dynamic shadows, improved character lighting, the ability to render hundreds of on-screen characters simultaneously, improved water physics, additional blood and bullet decals, advanced destructible environments, new weather effects, and the introduction of fluid objects. When combined with the sweeping vistas and evocative locales in the world of Sera where the Gears of War 2 story unfolds, these additions help make this the most gorgeous game available this generation. Gears of War 2 continues to push deep, immersive storytelling in video games to greater heights. Humanity's epic battle for survival is felt on both the personal and epic levels as you explore multiple story arcs that introduce new characters and flesh out the histories of familiar ones. A new in-game conversational camera system also helps bring the compelling story to life in a cinematic fashion.

100

Giant Bomb

The differences and enhancements in Gears of War 2 feel very calculated and intelligent. It's clear that no one set out to break things that worked before, and the most dramatic steps were taken in an effort to widen the game's appeal to players who might not be good enough to play it, otherwise. But it takes on this task without dumbing itself down and alienating the people who already liked the first game...All you really need to know is that Gears of War 2 is a terrific, kick-ass shooter with enough stuff in it to keep you busy for a pretty long time. 

100

X360 Magazine UK

A masterclass in design. Brutal, epic, beautiful and harrowing in equal measure, Gears Of War 2 announces Epic’s reign as the king of the action videogame. Absolutely essential. 

100

GameSpy

While it's certainly open to debate, we're going to go out on a limb and call Gears of War 2 the most visually impressive game of all-time. 

100

VideoGamer

It's not as emotionally deep as other games on the market, it's not a 30 hour + epic and it's not art in video game form, but is arguably the greatest action game ever created. In a year full of incredible games, Gears of War 2 sits triumphantly at the very top. 

100

1UP

As great as the campaign is, the game will see its longest legs from multiplayer. And from the multitude of competitive match types to the addictive and strategic co-op Horde mode, what felt like a hugely entertaining afterthought in the original Gears now easily stands shoulder-to-shoulder with the campaign. 

100

GamePro

Granted, Gears 2 isn't the smartest shooter on the market, but for now, it is probably the must-own title of this holiday season for Xbox 360 gamers. 

98

Team Xbox

All of the multiplayer, both co-op and competitive, completely makes up for any that was lacking in the first game. For those that thought that Gears just wasn’t suited to multiplayer gameplay, that it was too tactical and slow and was better suited to a single player experience, prepare to eat your words. It’s fantastic, and rounds out the complete package that is Gears of War 2...Gears of War 2 is about as close as you can get to the perfect gaming package. 

97

Game Over Online

Gears of War 2 delivers on every one of its promises, combining a rich and intimate story, brutal, visceral combat, incredible graphics, outstanding audio design, a strong soundtrack, and a robust multiplayer suite. 

97

3DJuegos

Gears of War 2 is the kind of game that third person shooters should try to imitate. We are not talking about the chosen theme or its tactical arcade scope, but instead we want to point out how well the console resources have been used in order to get an impeccable gameplay. A must have. The total shooter. 

95

Gameplayer

Gears 2 is a lesson in masterful pacing; whether it’s lulling you into a false sense of security, dropping you into an inescapable situation, or blowing your hair back with a ‘speeder bike’ vehicle section. It’s a ripping good yarn that is continually one-upping itself by belting you upside the head with yet another over the top action sequence that eclipses the last awesome thing you just did. 

95

GameTrailers

The game consistently manages to put impressive, holy crap-worthy stuff on-screen. Your ears get their fair share of candy, too. Voice work is almost always right on and the music is epic, foreboding, and dangerous. 

95

IGN

It has non-stop action, fresh gameplay, varied environments, an engrossing storyline, multiplayer that will have people busy for years to come, and some of the nastiest and coolest kills I’ve seen in a videogame.