Portal is a brilliant puzzle game that you can relax while you play it and enjoy the experience. The Portal chambers get harder as the game goes on, and the story line is great (linked to the Half-Life universe).
The Updated Source engine looks brilliant too, with lovely graphics to go with the game.
My only down side on Portal is its legnth, it will take the average gamer about 3-4hrs to complete, if that.
Still for this price you cant go wrong, highly recommended.