When in doubt.. think with Portal(s)
Portal 2 is almost undescribable. Darkly funny, fun, innovative (a biggy), and most importantly, addicting. Placing portals and solving puzzles is as fun as ever and looks better too, which a new graphics upgrade which looks awesome.
For those of you who got tired of the almost cookie cutter and bland test chambers, you are in for a treat because this time, you venture further out (and under) the Aperature Science testing facility. The puzzles are more challenging (but not altogether unsolvable) and the game is longer with a new and exciting co-op mode (thank god, the first Portal could be completed in less than 4 hours, which this campaign doubles, not counting the co-op.)
All of this with a reduced price normal to PC games, tremendous voice acting by J.K. Simmons, Ellen McLain, and expecially funny acting as the dumb as dirt Wheatley, voiced by that guy from the Ricky Gervais show.
So if you love games, puzzles, or cake (the references are missing this go 'round though) buy Portal 2 quickly, because there is science to do. And not just any science: Aperature Science.