I found myself coming back to this game in spite of myself, mostly because of the steampunk elements that caught my imagination. It's a good enough sh'mup game on its own, good variety in enemies and some levels are just this side of Bullet Hell in terms of challenge.
I agree with previous comments that this game deserves a story, maybe some cutscenes. The overarching goal of "escape the city" is good at first, but it needs more to keep me interested. Doesn't have to be a Pulitzer prize winning script, just something to give the player short term goals they achieve, which propel them on to other bigger goals.
For example, levels 1-5 are "Escape the City". After you beat the first boss, you get a cutscene where your airship is refueling in a free port or something. A mysterious stranger approaches the airship captain with a stolen letter that tells him friendly forces are regrouping in XYZ, but the airspace between here and there is heavily patrolled by enemy forces.
Levels 6 - 10 see you fighting your way to the regrouped army. There, you're given a special mission to help cripple the enemy's supply line and make their defenses vulnerable, allowing the army to recapture their city.
It's a simple idea, but something like that... a story that keeps the player interested enough to play the next five levels to find out what's going to happen, and then push them into the next five levels, would make a huge difference in getting players interested, and keeping them interested.
Otherwise it's solid. Keep on making fun games, man.