I always thought of this as a poor-man's Tiger Heli / Raiden. It isn't a great game, but you know what? I have certainly gotten some enjoyment from it. It really should have been a much better game, and I feel bad for folks who paid full-price for this back in the day.
Anyway, the one "unique" thing about this game is that your bullet stream sprays slightly to the left / right of your ship when you are pressing the respective direction on the direction pad. This is actually a neat idea to employ in a game where the "target" is constantly shifting and moving. I don't know if Avenger fully capitalized on this game mechanic (hmmm, it really doesn't), but I think there is potential in the concept.
Avenger is really a generic, lackluster shooter... but if you're like me, you'll be able to enjoy it for what it is (a bargain bin shooter). I love playing all shooters.
I think that some of the music is quite nice at times. Depending on your tastes, Avenger is worth purchasing just for a few songs.