First of all, it's important not to start watching this movie thinking it'll be the emotionally poignant and historically accurate. I actually think the title says it all. By the time you've heard of that movie called "Ninja Assassin", you already know if either it's going to be awesome or if it's going to be the worst movie ever.
For those of you who know a little about Japanese anime, this might be the live-action movie that comes closest to this genre. We've got a decent story that, in the end, just serves the purpose of tying together the bloody action sequences. The martial arts involved are well made and are surreal but not as much as "Crouching Tiger, Hidden Dragon" for instance. Ninjas are bound to the shadows and, in this movie, they literally disappear in/leap from them. There's a lot of CGI blood and cut limbs and it fits the concept well. Speaking of shadows, I first saw this movie at the theater and it's much better on a decent screen on Blu-Ray because there are many scenes that are very dark so the high definition really benefits this movie.
In the end, it's as bloody as it is entertaining and it has this 80's action movies vibe where you just turn off your brain and enjoy the show.