If you loved Halo, you'll love Halo 2 too. The game doesn't really offer anything new, which could be a good thing.
Or is there? Halo 2 has one extremely obvious advantage over Halo and that is Xbox Live. I was only able to play online for two months with a trial, and after spending about two hours downloading new maps and updates just so I could play, I quickly jumped into the experience. It was quite an experience which turned a bit sour due to some of the other players, but that's not the game's fault.
The graphics are quite a step up from the then impressive graphics of Halo. Master Chief has gotten a make-over as has many of his enemies. No real complains about the graphics, apart from the scenes shot on Planet Earth where the buildings look a little dodgy, but that doesn't really bother me.
The sound is incredible and if anything lives up to the hype, it's this. The music is quite fitting, and you'll notice some new versions of classics you enjoyed in Halo.
In conclusion, Halo 2 only offers some new weapons, characters, and one or two little features to improve on Halo, but it's still one of the best shooters you can get your hands on.
;)