It's a well-made story-driven puzzle game. The puzzles are pitched just right, requiring some head scratching but ultimately fairly easy once the solution clicks in your brain. And the storytelling's great, typical Valve - all first-person, no cutscenes, cinematic style, mute protagonist. It's another sci-fi universe, related to Half-Life (there are Black Mesa references tucked in a few out-of-the-way spots). There's plenty of exploration, good writing, and a strong visual style. Most importantly, it's quite funny.
It's just a prototype for Portal 2 though, which stratospherically elevates the storytelling, world-building and jokes. My opinion, Portal 1's pretty good, but Portal 2 is one of the very best games of recent years.
