It's hard to overstate how great this game was.
Before I bought it, I read many reviews, some of which were saying how everything about this game is awful, but some of which were saying this is a good game and worth your money. So I said, "Alright, I'll give it a chance, and hopefully it won't suck," and it was indeed above any expectations.
The main story is about as long as the original Portal, which is still short, but nowhere near as short as some people have said, and there's also single and multiplayer Workshop maps.
There is only one new mechanic, but it does change the gameplay very dramatically, and in my opinion the developers have made very good use of it, with fun and increasingly challenging maps that have different puzzles from those in Portal (2).
The narrator, I can understand why some people may find him annoying, but I like him. If you slightly lower your expectations from GLaDOS, he's pretty cool. He has a sort of theme going on that I would rather not spoil.
There isn't too much of a plot, but, again, it's fine.
I highly recommend this game.
(One last thing, you can tell quite a few of the more negative reviews are bulls♥♥t, because some people only have two or three hours of game on their account, which would never be enough to beat it.)
You can literally paint a ray of light. 11/10