A lot of fun, ended up breaking it pretty hard on easy mode. New PB Final Score: 20,672,950 ('score 544,000'), waves survived 38. At that point I was basically searching for angel deals at that point so I could make my weapons worse, so they would be more effective (they start to break a bit at high numbers). Eventually I just got bored and ran into a bunch of purple pellets, disabling my weapon completely and confining me to the corners and died soon after. (There's a limit to how much move speed / shot speed / fire rate? is actually good.)
Balance wise, the +1 bullet scaling would have been pretty strong, but it seems to work exponentially? So that's pretty busted on easy... though it does make the harder difficulties more playable. On the Angel side getting 50% health for -10% damage and -10% rof is pretty strong.
The interface art style and angel/devil deal parallels reminded me of Isaac quite a bit... Along those lines, there were certainly many times when I got a bad draw and would have rather skipped (maybe even with a small benefit attached like +10% health or +10% damage, for -5% of the other.)
Oh, and the powerup that says it bounces off enemies, never seems to actually do that... either it hits them and it destroyed from the collision, or you get piercing, and it pierces them instead of bouncing... It does bounce off walls though.
Music is quite nice, sound is a bit thuddy when you get overlapping / simultaneous bullet hits. Losing health for each shot is a pretty nice touch for the limitation. although it seems quite difficult to keep playing without grabbing the lifesteal, which sort of trivializes the mechanic if you choose to. The constant tradeoffs are very interesting in that same sense, though there are some that are definitely better than others, which makes for a nice bit of strategy I suppose.
Enemies are kind of generic, and some of them are difficult to distinguish from each other and some times it matters because they can have quite different attack patterns, Overall, very nice.