One of the best PS3 titles I've played in a while. I am having an absolute blast with this one, great setup, decent storylines.. I'm at the beginning, I suppose but it's very addictive!
I've completed both as High Chaos and Low Chaos. Frankly, completing as Low Chaos is a -bear-. There are some control issues at time (R2 is both block and choke/sleeper hold) and so there were times I'd find myself saving often in case I pressed "R2" and got the defense (which alerts the guards and then you fight it out) or sleeper hold.
Otherwise, the game and storyline is fantastic; the story is deep and well written, the characters are all interesting, and the concept is excellent. I enjoyed it a great deal, one of the better games I've played in a while.
It's single player. There is no online play. It's a longform story, you can play it through in multiple ways.. you can play it through as very good guy (killing no one, being unseen) or you can slaughter everyone. Most people will end up a mix the first time, killing few but not being unseen; the replay value is in figuring out how undetected you can go or how big of a scene you can make and still live.
I loved it; fantastic graphics, good voice acting, generally great story.
I love Fallout 3, but the open world, open questing gets me to burn hours accomplishing nothing at times. A linear game, could be a good change of pace. (one of the things I liked so much about Portal / Portal 2, was the clarity of story and game progression)