It wasn't difficult per say. Most of the time I knew what I had to do. I guess I shouldn't have played through it in one evening because towards the end I had a head ache and it was kind of unpleasant to figure out.
My favorite part was figuring out what I had to do in Portal, it usually wasn't too hard, just the right challenge. The painful part was setting up to do it and figuring out the coordination required. Portal 2 seems to put more emphasis on the setup and coordination and the part that was frustrating for me. But honestly, I'm not sure what else they could have done, they had to reinvent themselves somehow. I'm sure it'll be a great game, but I don't think I'm in a rush to play it. If I do, I'll have to do it in small doses... Once in 5$.