Very promising
I gave it an 8 because there were a few things I think could've been much better. Firstly, aesthetically, it was great looking but I think the levels could've used a bit more oomph in terms of looks and design.
Secondly, it would've been cool to unlock different hats and coats and whatnot. That didn't count for my rating though.
Third and fourthly, as they go together and are the biggies, the upgrade system could've used some work and the difficulty scaled a bit more with the guns. Now, what I mean about the upgrades is that you only have to buy a few things to unlock everything. It could work to the game's advantage to either let the player find upgrades along the way or work up through the guns and shields as you level. Which ties into the difficulty and scaling it according to level and weapons. Although, the way you did it was understandable. Maybe add 5 worlds next time?
Also, again, weapons, I think categorizing the weapons and evening out the power levels in each category would be pretty cool. Also, more variety of guns.
Lastly, more variety in music. Just a couple of more songs unless you guys would be up for individual level songs.
Great game overall, I definitely hope it has a sequel. You guys are one of my favorite developers around here.