At the risk of further derailing the thread, I think that the love of the Halo's stems from their multiplayer. Halos 1 and 2 were the definitive multiplayer games of the generation, just as Goldeneye was before that. It's smooth, well-balanced, and just plain fun. I'm sure if I could go back and total up the number of hours I spent playing multiplayer Halo or online Halo 2 it would be in the multiple hundreds of hours.
In terms of historical importance, what Goldeneye did to console FPS's and multiplayer, Halo2 did for online. Halo2's matchmaking service is absolutely brilliant and should be a model of how to do online multiplayer. It's widely varied, fairly ranked, and generally very quick.
I personally like the single player in the Halo games, but obviously it isn't for everyone. It has a great sense of spectacle - the set pieces just feel huge and intricate. It feels like a playing a blockbuster movie, I can't put my finger on what it is that makes it feel that way, it just does.
I know it's fashionable to hate on the Halo's, but it's a very good game. As good as the hype? Maybe not, but few "hallowed" games are (GTA and Metal Gear, I'm looking in your general direction).