Some say that Halo 2 doesn't come close to being as good as the first Halo in any way. I disagree. Though not as revolutionary, Halo 2 has much to offer.
The single player game is excellent. All the enemies from Halo are back including some new ones. I would have preferred a little more new enemies but its nothing to complain about. You can still play through the campaign with a friend via co-op which is always a positive. New weapons have been added as well. But apart from the alien sniper rifle I really don't like any of them. The assault rifle from Halo is no longer in the game and the pistol has been powered down as well. The grenades don't pack nearly the punch they did in Halo and you can't throw them nearly as far. However, Halo 2's campaign is just as great as Halo's.
Multiplayer is the best part of Halo 2. Now you can play on Xbox Live or play splitscreen or system link. The maps are more detailed and better designed for specific game modes. You can customize your characters appearance whether it be color, symbol identification or even if you wish to be a Spartan or Covenant Elite.
Halo 2 is a great game. Even if you don't particuraly like FPS's I recommend buying this one right away.