I enjoyed Halo and Halo 2, but I couldn't really understand the story behind the game. I think both are overrated though, I'm not that anxious to get a hold of Halo 3...I'm looking forward to GTA 4 and UFC.
Halo 2 kinda sucked since they completely ignored the single-player after a great single player campaign in the 1st one. The developers know they have a market and seemingly just slap together a weak 5-10 hour single player campaign. It's the "EA Sports" way of doing things.