Interesting game, but felt over before it even began. Some demos have more replayability value than this game does.
The graphics are those you would expect of most source engine games - - pretty good. The engine is getting a little outdated but valve have proven time and time again to pump out solid games using the same engine. The music in the game is very good, and accurately fits the situation the player is in.
As the player progresses through the game, the player realizes that there is more to the game than just solving puzzles. A humorous yet awkward storyline develops. Several times I found myself thinking "wow, this game is pretty funny." or something of that caliber. Although a solid storyline, it felt quite rushed and somewhat empty feeling as the game only lasted approximately 3 hours. It disappoints me because with such an original approach and unique storyline to a game, this could have been so much more than just one sit down in front of the pc.
In terms of replayability, it's basically non-existent. I've played demos that have more replayability factors than this, the "I want to come back for more" factor is long missing. There are a few advanced levels which don't take long to beat, and some other challenges that may satisfy some, but you're probably not going to want to play through portal again after beating it once. While the game seems intriguing and random one time through, its nothing better than linear the second.
But aside from that, this is a solid game and probably well worth your purchase if your looking for a 3 hour fix. If you're looking for a deep, difficult puzzle game that will takes hours upon hours to beat, avoid at all costs.