Weee!
That sucked away a bit of my time, and for the most part it was fun, but there were huge annoyances in it.
First off, that music. Make it stop! Give me a button to choose if I want it on or not.
Also check how often the bad guys spawn. It's not too bad when you're small, for you have a lot of space to move. But, when you're big (60 or more) and 5 baddies spawn in a row, there's little you can do but sit in the corner and hope you don't die. That's rather frustrating,
Similarly, when you get that big, the 'forbidden zone' is too big, even on easy. By 'forbidden zone' I mean any area where something can spawn touching you, thereby instantly killing you. It looked to me that on Insane, the 'forbidden zone' would be the whole map by the end. I would suggest either to stretch the playing area all 4 directions, or scale everything down a bit to give that freedom to move.
Overall, it was good times. These frustrations just prevented it from being great times.