At first when I got this game i was pretty excited for it, and until the end it kept that excitment. It has some funny jokes here and there, but this is a great game for fans of puzzle games. Even if you don't like puzzlers it has a great story anyone can love. Be warned this game is hard ( I had to use an internet walkthrough with some tests). Anyways i would really get this game I were you.
Portal 2 tries to keep the same level of interest in the single player as it had in Portal. However after completing puzzle after puzzle in dim environments or large areas it starts to become a little jaded. Not to say that it's a bad game, which is very far from the truth, just that the novelty and excitement from the first game does not fully transfer over to the second.
The highest point of this game is the coop. Being very unpopular I didn't have a friend on hand to play this with me in split screen or online at the time, I chose to try out the random partners online, all but one of which turned out to be very fun to play with making the game the best coop experience I have played to this date.
Overall I'd say this game is saved from the above average mark by its great coop system.