What is your favorite area/chapter?
Honestly, it's right at the very beginning. Wandering through test chambers I could recognize from the first game and seeing them all overgrown with plants and whatnot was so exciting. I love timeskip "apocalyptic" things in games and media... you know, seeing something and then seeing it again years and years later when it's broken and rundown and barely recognizable. :D It was creepy and awesome all at once.
What do you think of the co-op?
It's really fun to play with friends. I've only played it with someone in the same room, though. (My brother and I played it splitscreen and then a friend came over and played it on my computer while I linked up with the PS3.) I adore it but I also thought the plot was pretty blah. :( I'd have preferred it to be either separate from the game with no plot or as heavily involved as the main single player game but it just seemed like "here, have some pointless testing" and then suddenly "oh, here's a tiny bit of plot near the end". idk, it just kinda bugged me.