Finished Portal 2 tonight, yay!
It was every bit as good as I'd hoped. Fantastic visuals (Valve's Source engine is newly updated for the game) and Wheatley and GlaDOS were brilliant.
It's worth a second playthrough just for the commentary, the missed dialogue and the achievements.
If anyone's unfamiliar with Portal, it's a first-person puzzle game that must be solved by teleporting the player's character and simple objects using the "portal gun", a device that can create portals between two areas.
In "Portal " you play the human Chell who's just awakened from stasis after several hundred years. Chell must navigate the dilapidated Aperture Science Enrichment Center and its test chambers with the portal gun while the facility is rebuilt by the reactivated GLaDOS, a rampant AI computer. [Paraphrased from Wikipedia].
If you've never played it, get a friend to show you it or try the first Portal for yourself. It's available for cheap for consoles and through the PC 'Steam' service.