I got to wave 13 in the final level before I was overwhelmed!
The upgrade system is nice, I wish that I could keep my upgrades from one level to the next. The graphics are simple and clean, the sound effects are well suited to a space battle.
Enemy variety is nice, however every enemy and every fight played out the same for me, I'd circle left or right while firing and waiting for my missiles to recharge to take out most ships (or one strong ship) with that.
The game should pause when the upgrade menu appears, so that the player can make their choice without also being shot at. Sometimes I'd gain more than one level at a time and this would cause the menu to glitch out a bit, showing one set of rewards, then replacing them with others.