this is good, really good.
i loved the waves design, the power ups after each design, and the particle affects.
one thing i would like to note, is that, the plus sign enemies, looked like, they were some sort of token that you'd take. and not destroy.
besides that, really great work, everything is so clear, clear game, with clear concept and good waves design. and nice introduction of powerups slowly but surely. good work !
(also, the 5th wave is really hard :D )