Below average; the game does not have horizontal perimeters. IE: i can press up and just keep sending my spaceship into the sky, avoiding all incoming damage, indefinitely. I like the background and I think you should stick with a more detailed black and white game instead of having a decent background and the plainest, most boring sprites for the spaceship and enemy ships. Collision boxes are good, narrowly avoiding rockets was neat. Really poor display and 0 background information. A side scroller it is, but its just boring.
Recommendation: make the whole game black and white, refurbish HUD, (points and health bar), actually include bonus features, mmm donuts.