I am not a fan of Galaga-like games, but I liked that one.Sprites was great and gameplay too.But after some waves gameplay started to be boring, you rarely get hit by bullets and game becoming more easier.I think adding more fast actions(faster bullets, faster enemies, faster reloading) would be a great addition.Not counting that, game was fun to play.
Now I want to say about some bugs that I saw:
First thing I did notice is that when enemy ships are flying in, their bullet seem to not hurt player on collision (maybe it just was invincibility or hitboxes are too small).
Second: after player colliding a bullet(losing 1 life) sometimes he just appears out of screen(on screen borders)
Third: One time, after colliding with bullet the life of player wasn't decreased (or I just didn't noticed that)
Not counting all the bugs you did a great job on a game