It's a good game by all means, but at times it seems to get repetitive.
In case you don't know how the game works, you basically use this gun and shoot two different colored portals. Each color leads to the other. It doesn't matter what direction they are facing or how far away from each other they are. Momentum is maintained going through a portal. If you walk through a blue portal, you will walk out the orange portal. If you jump through a blue portal at high speeds, you will shoot out of the orange portal at high speeds. Get it? It's an innovative concept, and it works surprisingly well.
That being said, the novelty wears off. While still in the training missions, I found myself wondering if there was going to be more. Sure, it's a puzzle game. I know it's going to be a similar concept throughout the game. I just needed something...more. A twist of sorts. There are a few obstacles and the story moves along, but it got tired. GLaDOS is evil, we get it.
I'd like to break down a few things though:
Controls: They work well. The game is very responsive, and the camera never gets in the way.
Graphics: The environment is very plain. The textures are simplistic. The special effects are good though.
Sound: The fact that the "Still Alive" song is actually quite catchy is good, and enough said.
Replayability: Once you've beat it, it's not really something you're going to keep coming back to.
However, for the price of 1200 MS points, I think it's worth a try. You should get it just to experience the innovative play model. I do suggest this game.