This series will always live on in my memory despite my not having seen it for a number of years. Sho Kosugi was well cast as the ninja bad guy. It is such a shame that he never became a greater star, but such is life. However, the casting of Lee Van Cleef as the good ninja was just pitifully unrealistic. Whilst a fine actor, it required too much of a suspension of disbelief to believe he could do any of the martial arts maneauvers he was supposed to. He was just plain too old.
That said, I have many a fond memory of this series. No doubt, were I to watch them again now I would find many of the episodes cringe-inducing (just like some of the original Star Trek series). But still worth a look at on a rainy day.