Great except for a couple things
It's a fun concept, some minor improvements would be more kinds of upgrades and also to have the energy bar refill after each wave.
There was one major glitch in which I exited to the menu and when I started playing again I started over, but evidently something was shooting me and I had an invisible mini-coil. When I looked at the upgrade screen it said I had started from scratch. It appears that it's keeping some of the memory from the last game and that's causing problems. Other than that I love the concept and great game.