The GUI is essentially perfect as far as I could tell. A little bit of folly would have made the UI even better altogether (for example, during the level reset screen wipe).
Great concept, though the tracks don't look great and feel arbitrary (even though, on repeated tries, it's clear that there's a logic to them). I got stuck at the shot around the t-4 seconds on stage 3, though that might just be because I didn't understand the mechanics very well.