Portal is an excellent game. Fairly short, like Half-Life's episodes, but it's a clever blend of humor, action, and subtle and minimal storytelling. In that very Half-Life-like way, it starts out seeming like 100% gameplay with no story, and the story slowly emerges via gameplay, as the computerized voice you hear and the props you see knit together slowly to form a background.
Valve's creation of the Aperture Science Handheld Portal Device, or Portal Gun, was a genius stroke similar to their creation of the Zero Point Energy Field Manipulator, or Gravity Gun. Whereas the Gravity Gun grabs and launches objects with the two mouse buttons, the Portal Gun simply places red and blue portals. One leads to the other. So simple, but there's so much you can do with it. And Portal comes up with some crazy puzzles. One of the most interesting things is that when you travel through a portal, although their orientations may be completely different, (one on the ceiling, one on a wall) momentum is conserved. So if you place the blue portal on a floor and then jump off a cliff, falling 100 feet before entering the red portal, you'll fly out of the blue portal with momentum that carries you far up into the air. The possibilities are just crazy.
The bonus maps and achievements give the game a good amount of replayability. The bonus maps are divided into two categories: advanced maps, which are maps 13 through 19 with some element(s) added (or removed) to make it much more difficult. They're much harder than they seem. As of this writing, I have yet to complete one. The other category of bonus maps are the Portal challenges, which involve trying to complete each map in the least amount of time, the fewest number of footsteps, or using the fewest number of portals. The goals they set often seem ridiculous. I'm planning on replaying the main game at some point with the developers' commentary on and going for the Camera Shy achievement, which involves detaching all the security cameras from the walls.
Capping off the Portal experience is the wonderful tune I wrote about last week by Jonathan Coulton, entitled Still Alive. I've had the song running through my head for days now. Sometimes I swear I should retitle this site the Jonathan Coulton fanboy blog.
You can keep track of my Portal achievements at this link.