I'm not one for writing long reviews, so I am going to keep this short.
The game is really good, no doubt; the humor hits well and often, the old portal gameplay is made fresh again through the inclusion of gravity vents and gels, there is even a new credits song.
Despite all of this I completed the game in 6 hours, including breaks to make food and go to the toilet etc. The coop campaign is fun but once again, fairly short if you and your partner are good at working together.
It all comes down to how much money you are willing to spend on an experience that you could be done with in a day. If you are some baller zillionaire I can't recommended it enough, however if you are willing to wait it would be a wise choice.