Completed it yesterday. Played about half of the game in co-op, did the other half on my own. There were seriously bits where I just had to take 10 minute breaks and go around and look at stuff. So gorgeous. Started playing it on Insane yesterday, not as hard as I imagined actually.
Played some MP too, although I got really owned a lot in the beginning. It's fairly hard since people play extremely aggressively, and you cant really depend on outsmarting guys with tactics when they're storming into your face with shotguns blazing :G