The short version. It's fantastic.
I've been playing Portal 2 for the last two nights and the game is an absolute joy to play. It has a ton of polish and attention to detail. It oozes charm. The animation and voice-acting is so impressive that the game has an almost Pixar quality to it. Not in visual fidelity mind you, but in general look, feel, and level of talent involved in making it. All characters you interact with are robots, but their animation and voices are so well done they come across as more "human" than most characters in lengthy RPGs.
The game is also down right hilarious. Even funnier than the first one. I have not stopped smiling while playing and I have lost track of how many times I have actually laughed out loud.
The puzzles and level design is also fantastic. Solving a particular clever/tricky puzzle is quite exhilarating.
I purchased the PS3 version since Valve seemed to favor that one (over the 360). You get a free copy of the PC version through Steam with the PS3 copy. You can also play co-op with PC users from the PS3. And you get Steam cloud save support. I applaud Valve for again seeming to treat their customers right. The Orange Box was (still is) one of the best values out there and the PS3 version of Portal 2 seems to give a lot back. Good job guys.
Not sure how long the single-player is (I'm by Chapter 5 I think) but I've already put more hours into it than the first game. I'm in no hurry for it to end.