Awesome
Sure, it takes awhile to heat up... but once the game gets going, it's near-impossible to stop.
My only suggestion: a 'free play' mode where the user could select a level to start at (let us get practice at the higher levels!) without being able to submit score. It's a bit disappointing having to re-do all the early levels to get back to the challenge, and there's no reason to do so if we're not submitting the result.