Well, it's Portal, what do you expect? Brilliance from start to finish, with breakthrough performances from both Stephen Merchant and J.K. Simmons, as well as the return of GLaDOS's genius. The game manages to give us flashbacks of the initial surprise and charm of the first game, only to plunge us in something completely new and unexpected with some great new mechanics. The usual Valve ultra-polish is there, and the appearance of Co-op (complete with bombshell ending) is very welcome. If I have any criticism, it's about the lack of highly advanced levels in both coop and single player, although I imagine that will be fixed with DLC.
Anyway, like all Valve games, this is a must-play.
