The person that made this doens't fully understand how to make a run-'n'-gun style of game but does seem to understand the components that WOULD make a good one. Honestly, it makes for a rather dull experience overall. Let's start from the beginning so we can get an idea of what's wrong with it:
1) Enemy placement - Enemies are often placed in areas that aren't well thought out (or perhaps they are, but for the wrong reasons). You'll find yourself tapping the controller or keyboard to move forward rather than have your character move at a constant rate for fear of an enemy popping up on the very edge of the screen and immediately firing. While this isn't terrible if you are into that sort of thing, I like to play my run-'n'-gun style of games where I'm actually RUNNING and GUNNING, not stutter stepping and jumping.
2) Weapons - Good lord are the things generally useless. A bullet spray that shoots in a random barrage of bullets? Why not just make it the ever classic spread from Contra? In this, the bullets might miss smaller targets generally, and require me to maneuver in bizarre fashions to maximize the damage from an enemy.
A misslie launcher? Sounds amazing, except it's not because the missiles generally have this nasty tendency to fly upwards after a few moments of being shot so they always seem to skirt *around* the enemies rather than at them! If that wasn't bad enough, the missiles are incredibly slow as well. Lame.
Boomerangs? Okay, these actually aren't that bad, but it WOULD be better if you could switch between them and your standard gun.
I could go on and on about them, but let's just say that most of them are a noticeable downgrade from your standard shot. This is absolutely not good at all. Sure, you can have weapons that are better at moments, but having them be worst in almost any conceivable situation is absolutely unacceptable.
I especially love how the drops are random, so until you actually remember the symbols for your power-ups, you'll never actually be able to know which ones are actually useful or not (just skip most of them). Even worse is that some of them are a "partner" powerup, so there's no reason to not have those guys even if they have limited ammo and most aren't particularly helpful.
The Jet idea is pretty nifty but I feel like it's not utilized very well. Let's take the first mini-boss for instance, using your jet dodges every single attack it has, and even in hardcore mode, by the time it's actually ready to charge its next attack, your jet meter has already filled back up to the top again and can be used to dodge. I'm not saying remove the jet, but the enemies should have more attacks since the game knows that you will ALWAYS have the jetpack.
Lastly, I absolutely hate how the vast majority of the enemies are over your character's head but you cannot shoot up. That's not how you design something. Yes, I get that the jet pack is there to let you level out with the enemies, but it's excessive from as early as state 1, area 1-1. The helicopters should most definitely have a method of being shot at if you run underneath them outside of running backwards to shoot it (which causes enemies behind you to respawn), or running passed it; this makes it even more pointless to fight as it's no longer an obstacle.
All and all, it's not the most wretched game I've played-- it's not even really bad, it's just... Not good. In any real way.
Why was my review so long? I don't know, maybe it's a message to the developer so he can make a better game. I feel like if he made "Jet Gunner 2" and got rid of the above problems, the game might be pretty good.