A solid sequel, but one that does not break new ground.
With that in mind, Gears of War 2 is the clear heir to Gears of War's mantle as the premier third-person shooter and the best stop-and-pop game out there. With more weapons, bloodier action sequences, and stronger enemies, Gears of War 2 sets out to top the original in every way. It does, to some extent. But it doesn't quite have that same magical feeling compared to its predecessor.
Story (7/10): It's basically a mediocre action movie brought to life on the 360. I generally judge video game stories at a different level than I do those in other media, such as novels or films. Nonetheless, Gears of War 2 does nothing to distinguish itself even within the world of video games, where less is expected in the first place.
Little is done for the main character, Marcus Fenix, in this game. His main sidekick, Dominic, does gain some exposition thanks to an extensive plotline involving the rescue of his wife. But we never feel true sympathy with him, since it is quite obvious that the wife is just there to drive the plot forward and justify the movement from one setting to the other, from one mission to the next.
Given the larger-than-life villains in the first game, such as General RAAM and the Locust Queen, it is somewhat disappointing that you don't face off against such a formidable nemesis in this game. I guess they are saving the true pyrotechnics for the third and final game of the trilogy.
Gameplay (9/10): The mechanics of gameplay are essentially unchanged from the original Gears of War. Sure, they added some minor additions, such as chainsaw duels. But overall, it's exactly what you'd expect from a Gears game. You move from cover to cover, popping up to put down your enemies, and then moving on. Every once in awhile, you hop on a turret or do something slightly different that breaks up the monotony. As in the first game, unfortunately, vehicle sequences tend to be a bore - with the major exception in this game of a particularly thrilling segment close to the conclusion, which features a very different sort of ride.
How much you enjoy this game pretty much hinges on one question: did you enjoy Gears of War? If so, you'll like having a new campaign to fight through. As in the original, you have to muddy through a tedious and somewhat confusing story, but most of the time you'll probably ignore what they're saying to justify/motivate your actions and just concentrate on killing whatever jumps out in front of you. In my mind, there's nothing wrong with that.
Multiplayer (9/10): Gears of War was an online phenomenon, and I played it for quite a bit in spite of its shortcomings. The knocks on the first game were well-justified: everyone used a shotgun close-range, other than the occasional sniper/bow specialist hanging in the background, lending to a lack of diversity in gameplay and tactics. This was exacerbated by the general lack of game modes and paucity of maps offered.
Gears of War 2 has more game modes, new maps, and overall offers a similar experience. I think balance has been improved somewhat, and it will still suck in the hours. But don't expect a revolutionary experience. Personally, I think Halo 3's multiplayer suite remains king of Xbox Live at the moment, though I'm not a huge fan of that series by any means.
Presentation (8.5/10): Unreal Engine 3 is starting to show its age, believe it or not. I was shocked when I saw things in Gears of War 2 that seemed a bit dated or ugly to me, considering that the first game was undoubtedly the most visually stunning game I have ever played (with the possible exception of playing Halo on the original Xbox, which was similarly incredible for its era).
Some things in this game are just ugly, and the environments are not quite as impressive as Gears of War. Too much time is spent in caverns where one area looks pretty much identical to the next, diminishing the splendor that came with the "Ruined Beauty" aesthetic of the first game's city landscapes. Of course, you still fight in cities aplenty in Gears of War 2, so you'll see enough of that.
Overall, I recommend Gears of War 2 to any shooter fan, and to any fan of the first game it's a must-have. But don't look for a particularly long or replayable campaign here: it's 10 hours long, and you won't come back after completing it unless you want to get those achievements. Multiplayer is still strong, but I don't enjoy it as much as I did two years ago when it felt fresher. Pick it up, maybe as a rental, and you'll have a blast.