It's about as fun as you can get with this sort of arcadey, Tetris-like game. You drop small moons, asteroids and planets - which combine with other copies of the same planet/moon and create the next-larger sphere. Continuing until there is no larger object to create.

The celestial body theme is great, it's visually really neat and the polish of the animations (in particular the momentary delay before a new planet pops into being) give Cosmic Collapse a lot of life and energy. It's fun, but all you're doing is dropping planets and at some point you'll have had enough of doing that.. still a very strong little time killer.