Pure artistic genius.
After playing Portal one, a side project to the Half-Life series, I was a bit dubious about how Valve would introduce a sequel and how successful it would be. And lets just say I was MORE THAN IMPRESSED. This is by far the most rewarding single player game I have played in a while. This game makes you think completely outside the box - and the introduction of a story line and the new voice actors, along with the Original GlaDOS have truely made this game a winner. The incredible voice-acting and consistent dark comedic one-liners kept me laughing all the way to the end and really keep the gamer immersed.
The game just keeps on suprising you, challenging the way you view a presented problem. The single player itself can be clocked between 6-10 hours depending on the players problem solving skill level, but an additional 4-6 hour co-op campaign (picking up directly after the single player story line) is an absolute blast - especially on a system link at home with a friend.
I cannot express how highly I recommend everyone play this game. 5 STARS is an understatement!