So, having beaten both single player and co-op (with PenPen) here are some thoughts on the game - I'll try and keep it spoiler-free.
Overall I thought the difficulty wasn't really high enough for me. Both SP and co-op campaigns spend a fair amount of time introducing new mechanics and they don't really throw any puzzles at you that use all of them in interesting ways until the very end. I admit that co-op fared much better in this regard, though, mainly because you have to deal with twice as many portals with interesting restrictions on your characters' movement. You might say that the first Portal wasn't particularly difficult either and you'd probably be right, except for the fact that it also had Challenge Mode that let you play harder versions of some of the test chambers as well as attempt challenges like least number of portals used, least time taken etc. Portal 2 has none of that as of now, although single player and co-op challenge modes are supposedly going to be part of the first DLC.
That said, the writing and atmosphere are still great, and at least partially make up for the lack of challenge. The game is actually available discounted from many places right now, so if you can pick it up, I'd recommend it.