Portal is a innovative game with mind puzzles that is worth playing for it's originality and charm.
The goal is to achieve several tests in 19 chambers to defeat GLaDOS* and to get out of there. Very simple goal, but becoming clear as GLaDOS* tries to kill you through the chambers.
Gameplay: The game is really fun to play. Actually, it plays as a standard FPS (shooter) game, but it's more about logic. The character handles pretty well and the fun factor is there if you're a fan of action/adventure games.
Graphics: While the textures aren't as detailed as today's games, as the poly count and video effects aren't awesome, Portal looks great because of it's design. It's very clean and simple, technological-looking. It's sure among the modern styles, and I would not mind if this style was chosen in big companies buildings. The colors just go well together and the game looks great.
Performance: The game performs pretty well. If you fit the system requirements it'll work pretty fine. Works great on low detail on my integrated 128mb videocard (ATI X1200) in my 2 years old notebook.
Puzzles: The puzzles can be sometimes very hard, but hey, it's a puzzle game. Takes time to figure out what to do but it's OK. It doesn't manage your brain ;-) You'll sure get stuck, but if you really want to use a walkthrough, there is one on the internet for sure.
Bottom-line: With it's beautiful charm, GLaDOS* jokes, the FPS gameplay and the mind puzzles, Portal is really the only game like that out there and it's really worth your pennies. And the ending song, Still Alive is awesome! If I have a complain though to say about this game is that it's short and that there isn't a great story and no multiplayer maps where you could play coop with another person to solve puzzles that cannot be solved alone. That would have been very great. Now if only they could make Portal 2...
*GLaDOS is a real name, but the capital letters used as in the game mean something (OpenGL and MS-DOS are both part of computer software, openGL to render video images and MS-DOS for programming).