You know how I know I'm this guy? I recently played (and finished) Portal. Finally.
I'm not sure what would be gained from offering a "review" of this game. It's old enough at this point that you either know you should play it or have already played it. Everyone else probably just doesn't care about the FPS genre or PC games in general.
Still, I haven't met my useless blather quota for 2009 yet, so I might as well write about it.
This isn't a traditional FPS (first-person shooter, for those not in the know). The only "gun" you use during the game creates portals; by firing a blue portal at one location and an orange portal at another, you can instantly move through time and space. In essence, this is a physics platform/puzzle game played in first-person. It really is as clever and fun as everyone said it was.
I finished the game in roughly 2 hours. I'll give Valve credit for polishing it to a perfect shine, but it's essentially a demo, which is why I guess it came bundled with several other games as part of the Orange Box.
The mystery put together by the rather spartan clues of story in the game left me aching for more. Gee, what an unusual change of pace from Valve! The game should be enjoyed for what it is, but I'm a sucker when it comes to a good story, and the brief, leading clues drew me in easily. What I wouldn't give for a few minutes alone with the Valve writers just to pick at their brains.
Anyhow, this game comes with my recommendation, whatever that's worth. It's cheap and easy to pick up on Steam these days, too, so give it a try if you've got a couple of hours to kill and need something fun to do.