A must buy. Valve at it's best, even though the game itself is incredibly short.
Portal is a physics based game where you are controlling the Apature Science Handheld Portal Device which allows the user to create 2 portals and when you walk into one, you come out the other. This technique can be combined with the fact that if you fall 3 stories off of a ledge into one portal and place the other portal high on a wall, you'll fly a great distance out of the wall. A VERY cool concept.
This cool concept is executed perfectly over 18 levels that make use of most of what Portal has to offer. After the game is completed the advanced challenges and other challenges make the game MUCH more difficult and turrets are much more fun. Add to this the fact that user created maps that are even MORE challenging than the packed in advanced challenges, you have an experience that will last for a great deal of time.
Oh and the user based maps don't take up much HD space at all. About 1-3 MB per map, assuming the creator doesn't add any new major textures or art.
Overall, a must buy, even if you don't want the entire Orange Box the game comes with.