I downloaded and played this over the weekend. Absolutely incredible! There is more back-story revealed as you plum the depths of Aperture Labs. The humor is as great as the first one, and it definately felt like a much longer game. I did finish it in just two sleep-deprived sessions of undeterminable length, but that is more of a testament to its awsomeness than anything else.
The first Portal is broken into two distinct halves, testing and escaping. In the sequal, you're trying to escape the whole time, but in order to do that you alternate between the underbelly of the facility and several different groups of test chambers that lie in your path to freedom.
There are no time trials in this one. Instead, there is the co-operative mode which requires you to solve tests with another person.
And for the frosting on the lie cake, new Jonathan Coulton song at the end.
