It has been criticized for being too short. Two hours if you catch on quick, four if you ponder or multi-task.
It is short, but it's not too short. There is a big difference. I found that the game ended just as I was craving more resolution. To me, that's perfect.
I finished the game wanting more, but this doesn't mean it was too short, either. Merely that it was effective.
The writing is excellent: alternately funny and menacing without ever going out of character.
This is one of those games that shows every single strength of the medium. You could make a Portal movie, or a book, or a graphic novel, but none of them would have the same impact. The interaction makes the story a personal journey.
Also, the end credits are the greatest I have ever seen.