It is a beautiful game. Simply beautiful
The visuals are breathtaking. The detail level, the sand, the brightness... every visual aspect of the game was great, at least to me. It is (visually), very good.
The gameplay. The controls are fairly good, and the game is not quite open to crazy stuff; The only actions you can perform during the entire game are very few (move, fly for a short period of time, and a couple more), but, the 40 to 60 minutes of the story are definitely worth it. You'll find yourself without being able to take your eyes off the screen if you play the first minutes.
The multiplayer experience is very fun as well; playing with a complete stranger is really fun, and, although you will find yourself struggling to communicate using only a single button, it can be a really unique way to play.
The game itself is very strange, but that is a good thing. Personally, I liked the ending a lot, and, if you follow the story really close, I think you'll like it too.
¿Is it really worth $15? Definitely. Even if it is a little short, it is a very good game. I enjoyed playing it, and you will, if you like your occasional "philosophic" break.