The game is a little more "old school" than what I had anticipated, but I'm enjoying it very much.
After playing Ninja Spirit for 30 seconds, my initial reaction was
Oh no, I made a horrible mistake! The game seemed a bit stiff and rudimentary. You run up to people and hit them with your sword. I questioned all the praise I had heard about the game. (
Oh no, I thought--
could GoldenPhoenix actually have been right?!) I beat the first level and was pretty disappointed. However, after getting stuck on the boss of the second level for quite some time, I checked the Operation Guide and discovered that you have have three other weapons at your disposal! Suddenly, a whole new game opened up--one with lots of exciting possibilities.

I've found that the ninja stars are pretty useless. I don't see why anyone would choose them over the awesome power of the bombs. I was stuck on the second boss for the longest time trying to beat him with my sword. After I switched to bombs he was significantly easier (although still one of the trickier bosses). I haven't found the appeal of the grappling hook. It's slow and has a limited range. What's the point of this weapon? I've been able to swing it around a few times, but not with any reliability.
The ability to jump the equivalent of 30 feet in the air is fun--no one can deny that. I also like the shadow ability. Having two copies of yourself mimicking your every move can lay waste to bosses if you position them right. For example, on the boss of the first level, if you jump straight up and then don't move, your clones will hover in the air right next to the boss' head. You can destroy him in about 4 seconds.
The difficulty level seems to be all over the place. As I said previously, the boss of the second level is extremely frustrating because he can beat you in one hit. He flies around the screen and can be difficult to avoid since your jumps are so floaty. However, after beating him I got past the third level in a single life. The first half of the fourth level is the most difficult part of the entire game (so far--I'm only up to level 5). There's a really neat gravity mechanic, but there are loads of enemies who pop in that can kill you in one hit. But once you get past them the rest of the level (including the boss) is a cake walk.
Even though it's a sixteen bit game, I don't really see anything here that couldn't be done on the original NES. The animations are extremely limited--the walking animation for your character looks to be about two frames. The music is pretty basic. There are a good number of enemies on screen at the same time, which is fairly impressive for 1990. It might be a case of 'you had to play it back in the day'. It's a fun game, and I don't regret playing it, but it hasn't held up as well as some other games from the time.