I completed the game in less than 10 minutes. The music is one boring looped track, the only sound effects are the select button and the shooting; no boss or death sounds whatsoever. The designs are average, but this game has been out for two years and still hasn't been improved.
Great game, though as a lot of people have said buffing the boss would be nice (and maybe adding a super hard island as a postgame challenge). Add some troops before the boss perhaps. Other than that, a great 4 star game.