Portal is like Braid. And other small indy games that have a ton of polish and great style and exceptional game play. Gotta remember, Portal was like an extra that came in the Orange Box. Something most of us were actually buying for Team Fortress 2 and Half Life 2 Episode 2. Portal was a freebie, that absolutely dominated most pay games at the time of its release. Valve are geniuses at marketing.
I've beaten Portal 2 (at least the single player mode), and I liked it. A lot. There is a lot to love between the humor, the world design, the puzzles, and the world building. I'm eager to play more Co-Op, as it seems to be full of very challenging puzzles.
I did think the Single Player was a little on the short side, and lacked in difficulty in latter puzzles. But it looks like Co-Op is picking up the slack there, so Single Player remains a great cinematic experience.