To tell you the truth, I think that the soundtrack actually is worth the 4$ more than the full game is worth his 15$.
And you know why? Because it's a little short...Braid is cheaper, and ok, it's short too, but less then VVVVVV (and Braid is a fucking masterpiece).
Anyway, the gameplay is really addicting, and there are some stages (like
) that are really challenging. I love it