In a single sitting. Admittedly, this sitting was 14 hours long... (Playing on Medium helps too.)
The scale is quite impressive. Looking up from a (literally) city-sized hole in the ground, while standing at the bottom of it, makes every other set I've ever seen in a video game pale in comparison. I also like the new weapons, the chain gun and boomer shield were both welcome additions - though it bums me out that you can't seem to bring them with you across any barrier that requires a "vault over cover" move. With the bigger levels, hide and snipe works better than ever, and the longshot was even more effective (and more fun) than in GoW 1. (Edit: And the enemies still psychically "know" when you've got their head in your crosshairs.)
Still a lot of unanswered questions. Most pressing in my mind is, where has Adam Fenix been? He can't have been hanging out with the Locust the WHOLE time he's been gone.
I'm not sure if I'll ever play back through on Hardcore. The reaper chase sequence has the honor of quite possibly being the most annoying rail-shooter sequence in a video game, ever.