All done the single player. I think the best way to describe this game is 'charming'. The humour is impeccable, Stephen Merchant is perfect, GLAdOS makes you empathize with her while at the same time hating her.
There is literally nothing I would change about this game. My Steam clock says 7 hrs, and I would say this is some of the best 7 hrs I have ever spent gaming. Two or three puzzles slowed me down, but on the whole, it has bumped Portal 1 down a slot on my best games of all time list.
I really don't know how Valve does it. They are the best for a reason. The way they flesh out the side characters makes me empathize more with Chell. She never says a word, but I feel like I know her.
Which brings me to a sidepoint. On the PC Gamer Podcast 3-4 weeks ago, they had a discussion about whether or not Gordon Freeman was the best protagonist ever. The con side used the fact he never said anything and is essentially and empty shell as their argument. The pro side said basically the same thing, but argued that it's because he is an empty shell, you are able to put your own emotions and feelings in to the character. You're not feeling what you're told to feel (ala Call of Duty), you're feeling what you feel as a person. I have to say I found myself feeling the same way in this game. At some point, you come to empathize with Chell, and just want her to escape.
Next bit in spoilers, regarding the end game