Incredibly fun to play.
The graphics don't look as crude and the levels are repetitive. The new shield system and dual wield just make the fights more intense and action oriented. You still need some strategy to keep yourself from getting killed but you can now charge in and slice up enemies, which was really fun. The new missions are damn sweet. I just loved watching covenant fight with each other and having hunters back me up.
Of course, there is still room for improvement. You still don't have a map guides showing you where to go. However, it's not as much a big deal in Halo 2 because the levels are linear and distinct. I don't know how much time I wasted running around trying to figure out where to go in the first game. The new ammo system is rather tedious. I like covenant weapons so I have to constantly swap guns, which distracts me from the fighting. Also, the developers just ported over the xbox version and didn't bother adding new pc features like controlling various audio parameters, adding more keys, etc.
Overall, I would definitely recommend this game to everyone. The gameplay is just so much fun.