Overall, Ninja Gaiden II is what you would expect from Ninja Gaiden's sequel.
Graphics, Overall the graphics are terrific, with the occasional hitch here and there. The most noticeable decline in the game is the "worm cave" later in the game. The character models are sharp and look great.
Replay, This game is strangely addicting, even more so then the original. You are either going through trying to get the weapon play through achievements, trying a harder difficulty, or trying for the rest of the games achievements. The unlockable costumes are not quite up to the quality of the original, because they all are just color changes.
Difficulty, The enemies are just as difficult as they were in the last game, but this time there is something different. The developers thought it would be hilarious if they just throw more enemies at you. In the other hand, most of the bosses are easier, because on 90% of them you can just spam the dual katana on them to win. They also repeat bosses on several occasions which can get annoying because you already know how to beat them.
Stability, Some of the framerate issues are reasonable such as a famous stairway on chapter 10. Then, others seem like development laziness such as major slowdown by just walking. On a few occasions the game crashed on me, but that's rare.
Overall, Ninja Gaiden II is true to the series as being one of the best action games out there. It has lost some of its charm with the loss of a "hub" area for you to explore, some framerate issues and a couple of frustrating elements. Despite all of these downfalls the Ninja Gaiden series is still the best action series out there, I have high hopes for a remake.