I'm going to die a horrible, painful death for this but..
Portal 2 is mediocre average at best to me. Worse than the first game, anyway, but sequels are never better.
Portal 2 has a nice storyline, and of course so quirky, fun moments. It was also significantly longer, which could have been cut down a little for the sake of my short patience (but admittedly the length was perfect for the story advancement.)
The new gameplay elements are great, but near the end of the game it's like the designers couldn't make up their mind if the different puzzles should be hard or easy, which was stupid, it just kept switching between the two difficulties.
For me, it's worth it to play once, but I still didn't find it as OMG AMAZING as everyone says. Maybe if I ever get a buddy on Steam to play Co-op with me that'll make me change my mind on it, but that's doubtful. I'm pretty stubborn. xP
Edit: After reading over it again I realized mediocre was much harsher than I intended. I apologize for my bad word choice.