"Must have for xbox owners"
Anyone who liked the single player game from the first Halo will love the second halo's story just as much. Sure they should have spent a couple more levels on earth, but the other levels are still engaging, as are the levels you play as a covenant elite. 8/10 for single player.
Multiplayer is where this game really shines. Whether playing co-op, system link, or xbox live. Halo 2 is without a doubt the best Xbox Live game to date, it could also be considered to be the best Xbox multiplayer game as well. The servers are very stable and I have played 16 player team games with a mixture of NZers, aussies and americans with minimal to no lag. Throw in voice chat for team tactics or talking trash to the yank you just fragged and youve got the icing on the cake. 10/10 for multiplayer.
Bottom line, if you own an Xbox, buy this game now.