I like the concept and the execution but the bombs made this game super easy. After I had killed the second boss, I had all the upgrades except the extra ship weapon upgrades. And I didn't farm the first boss for more than maybe 5 times.
I see a comment that says "stupidly hard", lol :). Here's few tips: upgrade the money gain first, then upgrade the bombs. With 50$ per bomb, and 4 bombs, it will be smooth sailing from there on.
The bomb has a huge range when upgraded (covering nearly whole screen), and lasts for a long time (when upgraded). After getting the upgrades each boss was; activate bomb, keep shooting the boss, first bomb cools off and you have taken like half of the boss hp, activate second bomb and repeat and the boss dies without you taking a single hit since the bomb destroys all enemy projectiles. With bombs costing 50$, you will have 4 bombs every round even if you die in the first 5 seconds (which is impossible if you use the bombs).
So yeah overall I like the game idea, but the bombs being mega powerful, the experience was ruined for me.