Playing with a friend has never been this much fun. While the story of the single-player Portal 2 experience is what will draw you in for a few hours, it is the physical insanity of the co-op that will keep you coming back.
You and a friend, or indeed a stranger online, can take the role of two experimental robots who are outfitted with portal-firing guns that can connect two points in space. You need to use these, along with a host of other interactive tools, in order to get from the start of a level to its end.
Developer Valve recently announced a level editor so you will be able to enjoy user-generated co-op levels for years to come.