That was pretty fun. I'm kinda disappointed though, that wasn't difficult at all. I believe you tried to follow the steps of classic old-school shmup games like 1943, Gradius, Aero Fighters and such, but you missed the point of what made those games so hard and interesting to play, like the limited mobility, trade-offs and planning ahead factors.
The way your game is, there's not much to ponder. Difficulty increases just because there are more enemies to kill, but that doesn't make an interesting gameplay. You just upgrade your stuff, beat the enemies and save the bomb for the boss (which will pretty much kill almost every boss instantly). The bonus levels were pretty pointless too... (sadly, because boss rush challenges are a lot of fun, but then you put a sure-kill weapon there).
That being said, I might be one of those "real guys who like old style games", and it just seemed easy for me because I am very used to those diehard nerve-racking arcade/early console games. In any case I had fun, albeit honestly I was expecting more. :)