Many gamers routinely lose track of time while playing a computer game. For me, it has been years since it happened last.
When I finally sat down to play Portal, I finished the game thinking it was around 8 or 9PM, 10 at the latest. It was 2:30AM.
Some have complained that Portal was too short a game. Truth is, if it had been longer, I wouldn't have gotten any sleep that night!
Steve Williams has said most of what I would say about the game (all of it glowing praise). To his comments, I add the following:
If you are a game designer, or anyone curious about how designers make games, complete the Portal maps to unlock their Developer Commentary.
Listen to the Portal devs, and you'll see how they applied playtesting - and the principles of usability - to the game. In Portal and in Team Fortress 2 interviews, the developers have commented on how iterative design helped them make their games more usable.
Also, I think we'll be seeing a lot more songs written for/with computer games after the success of "Still Alive," the closing song to Portal, sung by voice actress Ellen McLain and composed by Jonathan Coulton.