Portal is a brilliant puzzle type FPS that uses some sweet gameplay mechanics to keep you thinking. Also, GLaDOS, the computer voice that talks you through your tests, kept me laughing most of the game. It has a surprisingly awesome story shoved into a game that really didn't need one, and I think the story is what makes the game excellent instead of good. The Weighted Companion Cube is another "character" that I have to mention as well. While I am not completely attached to it, I find it very amusing when you mix the box with a heart on it with GLaDOS telling you that it will never stab you.
I wanted to play Portal for a while, and I felt that, if I could sit down and beat it, I could wait a longer time to buy the Orange Box for myself. However, since playing Portal I want to buy the game more than ever. It is a weird sensation wanting to buy a game even after beating virtually ever puzzle in it. I did not do the extra challenges that extend the difficulty of Portal by limiting what you can use in each stage.
If you have not yet played Portal, I highly recommend buying it and enjoying it. It is available apart from the Orange Box right now on Steam, but it will also hit retail shelves on April 9th. While twenty dollars is a bit steep for a shorter game like this, Portal is not your normal game. Portal is completely worth the twenty dollars if you do not want to buy the whole box. Unless you want to buy the Orange Box, which you can do now, and get Team Fortress 2 and Half Life 2: Episode 2 as well. I however will wait to get the Orange Box on sale somewhere for my 360.
Portal gets a 5 out of 5 with value taken into account. It is truly a great game, though I would really be excited if they planned to release a much longer game for a stand-alone package. I would easily pay sixty dollars for a Portal 2 type game with 12-15 hours of play time for one play through. Though playing it would risk melting my mind. There is nothing wrong with that.