Great Improvement over previous game.
Let's talk about the campaign. The levels in this installment of the series are much longer than the original and that is a great thing. There is so much more to do and see in each level (not to mention many hidden things in most levels). Also there are new weapons to use and enemies to fight. In my opinion the Battle Rifle is a great improvement over the Assault Rifle in Halo:CE. Also the inclusion of Brutes and the bugs class (can't remember their names) makes sure that the gameplay always stays fresh and never feels monotonous.
The multiplayer is just simply revolutionary to the way other games will be played online from now on. There are many different variants of each multiplayer game type and that helps keep each battle fun and exciting. The ability to make custom gametypes will help with the freshness of gameplay and prevent you from ever being bored.
In the end Halo 2 is a great game, way better than Halo:Combat Evolved. I definitely recommend buying it and cannot wait for the next installment in this series.