Akira Kurosawa is certainly one of the most important directors who ever lived. Most of his most famous films were made in the 50s and 60s. Rashomon, Ikiru, Yojimbo, and The Seven Samurai may be the four most famous films he made, and they were all in black and white. That format was wonderful. His films had a definitive look in that era.
I would like to suggest, though, that he was the single best director of the color image who has existed thus far (whose work I am familiar with). I have only seen two of his color films (I don't even know how many he made), this film and Ran, but his sense of color in these two films is exquisite. I had to pause it several times during Kagemusha just to stare at the beautiful composition.
I personally think that Kurosawa's talents rested mainly in the technical aspects of his films rather than the content (and I'm sure many people would argue against me here). So as for the film itself, I'd give it a 9/10 for two reasons. I was only emotionally involved during small sections of the film (the end was particularly powerful), and the story was somewhat difficult to follow (I was confused during Yojimbo and The Seven Samurai, too). I prefer Ran to this film (and to all the other films of his I've seen, which include Rashomon, The Seven Samurai, and Yojimbo). Still, Kagemusha is very good.