Halo 2 isn't a bad game, but it's extremely overrated.
Honestly, it's the same old repetitive levels, environments and enemies as the original Halo. Nothing has changed, and there's nothing more to like.
If Halo left you wanted more, you will thoroughly enjoy Halo 2, because it's basically.. more Halo.
Once you fly by the game's short repetitive story mode, you'll find there's nothing left to do except for multiplayer, the main feature of the game. It has a great, beginner-friendly multiplayer mode.
The singleplayer campaign is a very straightforward, solid game that only just gets the point across, and the multiplayer of this game is the highlight.