The best overhead shooter of it's time.
The power-up system that they had was very well done to give players a chance when pitted against the game's challenge level, and it allowed for a little leniency when you made a mistake in the later levels. The boss fights were also very well done, making you develop different strategies to defeat each boss, and preventing the tedium that defines much of this genre.
The sound was very well done for when the game was made, having different music to set the mood for the boss fights and not relying on the same music overmuch.
Overall, I'd say it was an excellent addition to the overhead shooter genre and I strongly reccomend it to anyone who enjoys games from that genre and time period.