One of my favourite games ever. If you like challenging puzzle games you will love this. I wouldn't compare this game to portal, but I think those who enjoyed portal would enjoy this game.
Its challenging, has a great main story line, its interesting, unique and creative there is no other game I could really compare it to.
In parts the story line takes you dark and meaningful places, with the odd dash of humour.
Also this game is really long, there are so many puzzles its definitely worth its value considering the amount of playtime you get out of this, even if you speed ran it, it would take you longer than the average campaign.

Only negative would be that it gave me an extreme headache and feeling of sickness the first time I tried to play it, I enabled the anti motion sickness option in the in game menu and that pretty much solved the problem.