Great game, something new and innovative. Puzzles are fun and dialogue is occasionally hilarious. Extras not so much.
Puzzles are challenging while still being fun and addictive: giving up is not an option. Didn't take me that long to finish, not sure about other people: My two complaints are that it's too short and that the extra challenges you unlock after you beat the game are pretty uncreative and lame.
The dialogue, provided by your trusty guide "GLaDOS" (Genetic Lifeform and Disk Operating System) is often ironic and humorous. Easy learning curve, not an easy game. Worth the money, forsure. Look forward to Portal 2.
Definitely well worth the 20 bucks.