As many had already said, it gets really tough after the 4th or 5th level. It burns the fun too fast, turning it into desperation.
The tunes are nice of each bubble are nice, although, if you could stick a melody, based on those tunes, it would be even better.
Just right when it becomes really fast, it reminded me of my little cousins with one of those little pianos for kiddies, or another toy similar to those, that have a little hammer to hit some metal pieces like a marimba.
 "Touch The Bubbles"){kind=icon}
 "Cosmic Crush"){kind=icon}
 "Age of Japan"){kind=icon}
 "The Missile Game 3D HS"){kind=icon}