This was a triumph.
My daughter gave me the Orange Box for Christmas, and I just finished Portal.
One of the greatest game experiences I have ever had. Portal really gets into your head, like a good novel. I love that in a game.
But the true stroke of genius was the end sequence. Too often you play through a game and are left feeling empty when it's all over.
But the ending of Portal was the most rewarding ending of any computer game I've ever played (and I've been playing PC games since 1985.)