Despite the fact that ninjas can do all those cool thing we mortals only dream of being able to, I agree with the others... If you add just a tiny bit more bending to the knees, and some more swing in the arms to help the flip, I think it'll look a lot better, more natural...

Don't get me wrong, I think it's a good animation, but he looks a bit stiff to me, which, I think, takes away from him being a ninja...