Worth it just for the Comedy...
Portal 2 sets off from where Portal left off (killing the evil AI), this is where you meet a new AI, and the game is worth buying just to listen that AI talk.
In terms of graphics, they are good but are nothing to write home about as a lot of PC's will run it on high. The puzzles gradually get harder and more frustrating and just when you think oh this game is way too short you're wrong as it continues to grow on you (you'll know what I mean when you play it)
The good thing about Portal is that it is a game i'm sure anyone who plays it will enjoy it
I have not played a really funny game since time-splitters came out, the comedy in this is decently on par, worth buying. I will test out out co-op at some point and post on it.