I've not actually got the game myself, but I did go to a friends house recently were a few of us were taking it in turns to complete the Co-op mode. In my opinion, the Co-op mode was amazing, it's one of the greatest multi-player things that I have played in a long time, the humour in it is great and working out the more difficult puzzles was more than satisfying. From what my friend who has completed the single player campaign has said, the single player is amazing and more than worth the money that he paid for it (although he didn't think that the wait since the original Portal was released was justified in the game). He said that the writing throughout the campaign is awesome, and that it just gets better and better the further you progress in the game (I don't want to go into any more detail though, as I may end up spoiling it).
From in the Co-op mode, I can tell you that the puzzles are great, and more challenging than the regular puzzles in the original Portal were, so I can't imagine what the puzzles in the campaign were like, but they are more than likely awesome.
It's an amazing game though, I'd love to have a copy myself but unfortunately, I don't have the money to acquire it at the minute.