Never got to use bullet time cause the only things you need to do to reduce the danger is to stop shooting, and altogether I'd say that the player creates the difficulty himself, which is an interesting idea, but I feel like it makes game a lot easier. Maybe making the character shoot all the time would be a good idea? Perhaps, as a second game mode?
I also feel like the difficulty spike when you upgrade to next level is rather steep, I think it would be better if upgrades occurred more often, but were more gradual.
Overall, game is fun and the graphics are an eye candy.