Ninja Gaiden Dragon Sword is definitely one of the best games I have ever played.
I would advise a screen protector when playing this game. Actually, the first thing anyone should do when buying a DS is invest in a screen protector. The touch screen gets scratched very easily.
The difficulty of the game on normal mode is pretty easy. Although, I did notice that it got even easier when I was using my special stylus that I could wrap around my finger. I could slash at the enemies on the screen a lot faster and easier then when using the normal stylus.
The bosses were challenging enough to make the game interesting, but not so much that I wanted to throw my DS across the room.
I was able to beat normal mode in 8 hours, so its not a very long game.
If you want a short, action packed, and amazing game then Ninja Gaiden is definitely recomended.