Bottom Line: Your Xbox just isn't an Xbox without Halo 2.
The single player is simply awesome. Despite what some criticts have said. Well, ok, I played Halo 2 first, then I played Combat Evolved. Thats why.
You'll go through several levels, like the Cairo Station, Regret, Delta Halo etc, and you'll basically be shooting your way through waves of Grunts and Elites, occasionally you'll run into a few brutes, and hunters. It can get repetetive, but it's just plain fun blowing things up. A few levels will be played through the Arbiter, which is also fun. Co-op is always a delight.
The weapon selection feels really balanced, especially in Multiplayer.
Speaking of multiplayer, everything feels really balanced. And the dual-wielding, while is nothing new, is basically what makes the game feel balanced. Other games have dual wielding, but don't have the balanced feel that was accomplished in Halo 2. No matter what combo you have, wheather it be a SMG/Needler, or SMG/Plasma Rifle, or SMG/SMG you always have a fair chance at getting some kills. And unlike in Halo 1, you won't be able to just get by with a pistol, which makes things fair. All the maps are really cool, especially if you have the map pack. Overall, the multiplayer is obviously too much to be sumed-up in one review.
Halo 2's graphics are well done, for the most part. There are a few instances where you'll see pop-in and some slowdown. But given the scope of the game, and how much fun you're having, you won't really notice it. Normal Mapping and Specular Highlighting are everywhere in the game. And that's about it.
Halo 2's Audio is no slouch either, offering a more cinematic flair than the first game did. Gunsounds, and explosions will rock sound systems, if you have a 5.1 setup. The cutscenes sound well constructed, and the voice acting isn't that bad either.
Its pretty obvious that Halo 2 isn't the greatest shooter ever made. But it is still the best shooter available on the Xbox. The Bottom Line is: Your Xbox just isn't an Xbox without Halo 2.