Cool game, I was confused initially because I was expecting the graphics for the balls in the tutorial to be similar to what they were in the game, but it was fun once I figured it out. I like how it builds up and you want to maintain a reasonably circular shape so you have more room to maneuver. Would have been nice if you could rotate the whole thing.
I felt a little constricted to staying in the middle of the play area because there was a possibility of a red circle spawning in and flying straight at me, with no time to dodge. You could have the new circles fade in on the outside, and maybe even draw a trajectory for them.