I've just completed the singplayer. The last stage of the last level was quite hard.
My final verdict is that this is extremely solid single player. I really can't remember when was the last time i played singe player as good as Killzone 2 (and yes, i've played CoDs, Resistance games, Halo etc.). Orchestral music is really amazing. Cutscenes look brilliant. Lighting is especially really well implemented into them and it shows.
There are few FPS drops here and there, but FPS remains solid during the most of the game. Of course, game is, viewing from the pure graphical PoV, best thing seen on consoles and i really can't even imagine what's going to beat it in the graphical department. Something from Sony's 1st party probably.
As some of you have noticed, there are some bad textures, but overall, there's really little you can say against graphical presentation of Killzone 2. It's simply breathtaking. I've said this before, and i'll say again, DO NOT judge the game based on your experience on the demo. Demo really doesn't show the beauty of this game.
AI is great, they adapt to different situation really well. There are some bugs, but nothing serious. In contrast to Resistance 2, where you were the only target of enemy attacks, your comrades will be under attack all the time.
Total time it took for me to complete the game was little above 10 hours, though my friends who played on Normal (i played on Hard) managed to complete it in around 8 and a half.
There are some quite interesting trophies in the game btw, and their icons are really nice.