I actually got the game for the multiplayer part only (and I love it).

But I loved the single player mode too. I haven't played much of it, but I watched Seamus (SeanT) play through the whole thing and it was awesome. I'll probably enjoy playing it less than I enjoyed watching it, but that's only because I get stupidly frustrated when I know what to do but can't, and if Portal the First and the co-op mode are any indication, I'm more likely to get stuck because I'm unable to do the deeds than because I can't figure it out.
Seriously, Portal was for me an accumulation of doing the same thing fifty times (or more) in a row until it finally worked out the way I expected it to. Good thing I'm insane.

I also agree that single mode knew to change things up before they got too boring, which was appreciated. Seamus found it too long, but I think that's only because when he got closer to the end, he didn't want to stop playing it until he was done. At that point you lose some of the enjoyment because you get so desperate to finish so you can go to bed.

I can't wait for the DLC, though, because I definitely liked the advanced and challenge modes in Portal and I hope they make a comeback.