I have been studying kuji-in for a few years now, and it greatly enhanced my martial arts experience, but also my life in general. This is a sacred science that is sometimes thrown away on websites, only picturing the hand position and the quick mantra (RIN, KYO, TOH, SHA....) but i found a school that introduced me progressively and then transmited the entire ritual with some guidance and phylosophy. There is much more to kuji-in than what we see in ninja movies or anime.
There are around 3,800 schools and traditions of kuji-in, and each have their little variations according to each their own experience, and a bit of language translation after thousands of years, but the main practice is the same in all. I studied with the Dragons of Justice (the kuji-in school, not the anime characters) in Canada, but i did my contact on-line thru the web.
The physical strength, the quickness, the flowing adaptation, the awareness of the self and environment,.. are only side-effects of the real kuji-in experience. It is said that the teachers of kuji-in keep their secret intensely, but i beleive they don't have to work hard to keep it a secret, since it cannot be transmited by words. You have to learn the ritual basics, then be guided thru the experience, and then Voilą! There is a new consciousness about things, a new sight on matter, a feeling of "I am spirit within my body".
I recommend learning the kuji-in basics to anyone, and i trust my school, the Dragons of Justice, to be quite efficient at teaching it. I hope sharing my experience can bring some insight to someone.