Seems like we both made games with time shifting mechanics :P
I do really like the core idea. I think I have a few nitpicks, though. I think the combat could be more satisfying (maybe faster bullets, hit-stop effect on hitting an enemy, maybe more of a crunch sfx when hitting an enemy). But I really had fun, and I like a lot of wave-based games. I really loved being able to choose my own upgrade path.
Really good work, and, again, cool concept!