I love the guns, the level design is fun, and I loved the retro graphics. It's a really good shooter!
There really isn't too much to complain about here, except the upgrade system; that needs to be more robust. I really wish one could respec in this game, especially because some of the upgrades are quite terrible. So terrible in fact, that it would be better to leave the weapon as default rather than upgrading. Once you do upgrade, however, that's it, there's no going back or selecting the other upgrade.
The only other problem was how easily one can die from falling in some of the later levels. That was quite a pain in the butt.
Other than that, I would say this game should get a 5/5, but the problems that the game has are just a little too big to ignore. 4/5 is therefore what I said. It's good!