Mediocre
The levels were rather uninteresting and repetitive with no obstacles other than enemy ships. That's fine in games like 1943 or Twinbee where there's tons of bullets to dodge and loads of powerups to collect, but since none of that is going on here, it leads to a very boring experience. No one level really stands out here as well- especially with the same music. Every level of every Gradius game is extremely memorable and, ergo, can be mastered through repeated playthroughs.
The art style, though intricate, seems very flat and sterile. A lot of games in this genre aim for bolder colors and higher contrast on the foreground objects to not only make the game more visually appealing, but also to make hazards more noticeable. None of the ships (including the player's) were animated aside from the thrusters.
Music was okay, but did not loop smoothly and got repetitive quickly. I eventually had to mute it and put the awesome Xexex soundtrack on ;).
Lastly, the hit-box on your ship is, in my opinion, far too large (the Vic Viper's in Gradius V was like, one pixel in the cockpit). Pulling off some extreme maneuvering through enemy fire is half the fun in a game like this, and it's neigh impossible with the hit-box as large as it is.
Having played a lot of the old-school shmups, (my favorite genre of video game) this one was just 'O.K.' Wasn't terrible, but doesn't really stand out.