Halo 2 is way better than the first one, the single player campaign is much more exciting and intense, plus the way they tell the story is much more ahead of anything Halo 1 did.
I don't understand all these people that say they loved Halo 1 but don't like Halo2, that just sounds crazy to me...I don't know, maybe it's because bungie took away your god pistol. The riffle is a much better alternative anyway. I will agree that the game is too short though, but I'm replaying it now on Heroic and I'm about half way through, and it's a much better experience on the harder difficulty levels.
As far as multiplayer, I can't get enough of it on XBLive, I never had bothered to purchase live before, but this game came with a free two month trial, so I gave it a shot, and wow...perfect, I think I'll probably keep my subscription after the two months is up just because of how much fun it is.