Don't believe anything they tell you about the cake. It's a lie.
This game definitely is a must-have. I really enjoyed (and still enjoy) playing this. The storyline itself took me about 2 to 3 hours to complete. The additional challenges and advanced chambers are nice features, great for adding some game time and a challenge for those who like getting all achievements.
Through the game and story, there are several nice additions (random radios, messages from other test subjects) which caused me to laugh from time to time. The chambers and puzzles get harder smoothly as you advance through the game, which goes from okay-this-is-easy to omg-how-am-I-ever-going-to-solve-this. I really love the smooth physics. Good graphics, the music and sound are okay.
I definitely recommend buying this game. When it's in sale, I recommend buying Portal 2 too, if you have some spare money. Two games you just NEED to have in your inventory.