This game has been out a while, but I cant help but gush about it. As far as a puzzle game goes, it is incredible. It is mind-bending like Prey, but in a different way.
Then sense of humor pervading the whole thing is genius. I love GlaDOS, she/it is easily one of my favorite in-game antagonists to date.
I am running a week long tribute to Portal in the holiday spirit over on my blog:
http://popcultureobservations.today.com/
Feel free to comment. there is also a linkt here to a 2d flash version, which is actually very cool.