Looks like my review will be the first :-) You definitely know how to make games. The graphics are great, the physics and controls work 100% fine for me. Being able to crash all of these crates - priceless. like Duke Nukem would say: "He he he he, what a mess!" ;-) I only didn't like few things:
1. Probably the worst thing: the information board about the boss near the boss zone, when the camera zooms in and you partially lose control of the shooter. I even got killed once because of that. You can leave it, but without that camera zoom.
2. I also have a feeling that for few seconds after the board (this one or initial) is being shown, the game works slightly slower.
3. I don't like the fact that most of the times when I have to jump down I don't know anything about what is waiting for me, what opposition will there be etc.
4. Make found hearts collectible only when the health is partially depleted.
5. I assume that it was intentional to make the player unable to kill all the enemies once and for all since they respawn, but it irritated me when for example one guy respawned just in the place where I was currently standing and costed me a heart.
6. Sometimes I will really wish that I could run fast & shoot at the same time.
7. Maybe besides upgrades, enable also some legendary guns to buy? Like Peacemaker, or maybe Dirty Harry's Mr Smith and Mr Wesson (if I remember that correctly) :-)
Anyway, good job! :-)