Reminds me a bit of my game actually! 

I've enjoyed it! It's quite difficult as others have mentioned, I've played a lot of times and am really struggling to get past the 2nd/3rd level. But the fact I'm still playing it is a good indication of the fun to be had with it! The variety in the enemies was cool too!

I liked the choice of powerups after defeating an enemy, that made me replay it quite a few times to see what they all did. However, because they seem random, sometimes it was brutally hard if you went into the 2nd level with only the starting pistol (because the previous powerups were just health or clock).

I was less sure about the choice to give health back to the enemy when you rewound though; that to me somewhat negated the rewind ability a little bit. Because of that I only felt compelled to use rewinding when I knew I had gone through a stretch of taking damage and not damaging the enemy much, or if I was about to die and had no other choice (even if that meant the enemy would gain most of its health back and i would effectively be starting the level again). 

One problem, which is the same problem in my game, is the lack of a continue button when you die; would have been nicer to just respawn on the level I was up to with the same powerups (but I ran out of time implementing the same thing in my game, so I cant complain too much :P ).

Overall I quite enjoyed your game!