Really fun. I had some problems in the beginning with finding smaller particles and seeing if I was larger than another planet already. Tried to took in one that looked smaller, but it still killed me. In later levels, I sort of systematically searched the universe for particles and then headed for the center to take in the last few. I was thinking, maybe a sort of radar screen hud showing where you are and where the largest and some smaller planets and stars are would be useful. Especially when you're becoming quite large yourself, you can't just hover next to another planet to see if it's larger than you anymore.
Also I kinda expected after the Planet-Sun level, I'd now be a sun trying to become a galaxy, instead it just turned into larger stars. I think a galaxy could be cooler, after all, the main reason to keep playing is to see what's next.
Still it's a fun game, the lack of timer is fantastic, it doesn't need to be all fast. Also when you die you just start at the same level again, that's good.