I will be shocked if Halo 3 is not an XBox 2 launch title.
That being said I think Halo is grossly overrated and I have no intentions of buying Halo 2. Half Life and it's many iterations make for much better LAN games. Console shooters are somewhat inferior in general, and Halo specifically gets really boring. Vehicles are over-powers in multiplayer, and the campaign is overtly repetetive.
Half-Life 2 will own Halo 2 all day long. And Half-Life 2 will have a number of strong mods to increase gameplay because it was built and designed around a SDK and a heavily moddable engine. Playing Halo today is no different than Halo at launch. Halo 2 doesn't even have all the features that were promised in the initial launch of Halo.
Why does everyone worship this game? I assume it's largely a younger crowd who doesn't have a good gaming rig and was first really broken into the FPS scene with Halo.
I'm not even a big fan of the FPS scene, but I've been playing them all since Wolfenstein on my 286.