Well, I'm late to the party but I just finished the Singleplayer campaign. Five hours (or how Steam registers it, it could be 5h59m for all I know). It's definitely worth every single buck I spent on it. It's just as chock-full of fantastic dark humor, great puzzles that are just good enough to make you scratch your head and 'explore' the levels, while still remaining well within the frustration limits so they're fun puzzles instead of frustrating.
The characters are great. Wheatley, Cave Johnson and as always GlaDOS is easily the coolest depiction of an AI I have ever witnessed. The level design is fantastic, really. The way the levels move and the sights you see while in transit... I am personally amazed at how well Portal 2 has evolved since the original. The exact same core gameplay, with added hilarity, more epic levels, more story, more complex yet still intuitive puzzles...
It's worth every dime.
And I haven't even tried the coop yet.
Edit: Also, what's the statute of limitations for spoilers on this game? That ending? Come on!
Lemons! I'M IN SPACE! Let's talk spoilers!