Great game!!
Even though Halo didn't revolutionize the first-person shooter genre, its influences have already taken place -- it's two-weapon, one grenade system has had profound effects, just to name one -- and it's earned its place in the annals of gaming. Halo 2 is similar in form. It's not going to revolutionize the world of first-person shooters, but it stands on the shoulders of other really good FPSs, and it improves the game on every level.
The best parts of Halo 2 are subtle. No one thing stands out and smacks you on the face and says, "LOOK AT THIS!" Instead, the whole product, the entire game, from top to bottom, is remarkably well produced, well paced and planned. Very few games come together and look, sound, and play as smoothly, organically, and as well as this. It's got dozens of exciting, intense gameplay segments, excellent graphics and sound, and though it sounds like a cliché, the game is filled with millions of the little things that accumulate in your subconscious. The end-result is that it's not only addicting, it's not only the most fun you'll have in single-player or multiplayer game on Xbox, the story is deep and smart, and it keeps you interested. Perhaps best of all, the online portion will essentially blow everything else on Xbox Live out of the water. In a way, there are dozens of surprises about Halo 2, and yet there none. It is the best Xbox game this year, and it is the best Xbox game on the system, full stop.