It was fun and hilarious. I played til' i started getting dizzy. But a masterpiece? Not even close!
The gameplay on the other hand isn't actually that impressive and I don't really get why the puzzle solving in Portal is so hyped. I thought it was easy. Sometimes very clever, sometimes just plain and dull. I felt a rush going through the levels. I wanted to know more about the story, and in my way was these sequenced puzzles with annoying loading screens separating them.
At one time I thought the gameplay was really going somewhere, when a sequence of "running away". After that followed what I consider the most fun part of the game. The part where it didn't feel like a series of tests, but rather a big challenge. A mission. I wanted more of that, more of the gameplay outside the boxy, sterile rooms.
Story/writing: 10/10
Atmosphere: 10/10 (Except for the sterile white rooms, which I find quite boring. Luckily there's plenty of the non-white rooms in Portal 2!)
Gameplay: 7.5/10
Visuals: 9.0/10
Innovation: 10/10
Humour: 10/10
As you can see, it pretty much nails every aspect except for the gameplay. But I guess the gameplay is very much individual, and if you happen to enjoy it you will probably love Portal 2. If you're like me who wants more complex puzzles and different ways to approach them you could buy the game just for the lolz and compelling story.
Oh well, try it out and find out how you feel. I feel that it is a little overrated just because it is Valve and just because of the innovation. Gameplay wise it isn't anything special, and that is probably why they have put so much effort on the presentation of the game.