Average
That's really all there is to say: average. The sound effects are nice, and the graphics fit, but the overall gameplay is just boring. Your ship moves way too slow, not to mention it gains momentum, making it unable to stop on a dime. That's a no-no in space shooters. Also, you have all these great retro sprites and sounds, where's some music? How about burning heat from gradius II or practically anything from raiden fighters.
There's also the fact that there's only one type of enemy, and it doesn't shoot back. You had a boss, but that's not enough. Have a longer stage with some basic foes at first, then shooting ships for a while with the basic foes, then the boss. That's called pacing. Finally, how about some power ups? After all, that's what makes these games really fun. Follow these tips, and you'll have a decent SHMUP on your hands.