It's a great movie, doing exactly what it was meant to, so I saw no reason not to give it 5 stars. Still, you need to know exactly what it is you're going to see with this movie.
Fighting, action, mystical ninja powers, dismemberment, bloodspray, etc. That's the entire draw of this movie and it is done so well. Right when the story starts to coast the movie flashbacks to some backstory, and by backstory, I mean a glimpse into ninja training or awesome fight scenes.
So, this is a guy movie to the max. There's a plot, it's there and tangible, but it's a guy movie plot. They didn't need it but it's nice they included it. Better just to sit back, relax, and enjoy gratuitous amounts of animated gore.