No comparison here.
Half-Life 2 by ten -fold.
Disadvantages of Halo 2 are that first of all its on Xbox. Its limited to a lot by just being on Xbox. Graphics, Controls, Envorniment, and lack of a new engine. Xbox can only take so much. It's only a 733MHz and a Geforce3 based card. PC games are just -more thorough and more defining.
Besides all that, Halo and I'm assuming from the movies I saw, Halo 2 are good ol' shootin' purple alien fun.
While that might be appealing to some, I'd much rather stick to HL2. Halo 2 will have its moments, I'll give you that. But no way in hell it'll live up to or let alone be in the same category as Half-Life 2.
I can tell by just reading all of your responses that you aren't PC Gamers at heart. I started as one. Half-Life redefined the FPS genre as we know it and PC gaming for that matter. I suspect Half-Life 2 will have a similar impact.
Just my two cents.
obscure detour