I'm bursting with good things to say about this film. Japan's (maybe the world's) greatest actor playing one of Japan's greatest heroes is already goosebump inducing. In fact, all of the cast is stellar. As the other comments have pointed out, if you don't know the story of the Shinsengumi, you're at a disadvantage. This is a sort of Japanese Iliad, a real life story of heroism and tragedy. Everybody in Japan knows the names of the leaders of the Shinsengumi in the way that American children learn about George Washington and Benjamin Franklin. The good news is that the DVD program notes are fairly encyclopedic. They cover the major characters, places and events. Serizawa's entry mentions that "his hobbies include drawing, long fights on the beach and driving foreign invaders from his homeland."
The film starts out well by including the ugliness of the Wolves of Mibu period. Hijikata is portrayed in a very dark way. Toward the end, Kondo actually says that Hijikata did all the dirty work so that Kondo could keep his hands (and soul) clean. Even though the Shinsengumi are the heroes, there is a point where you will be yelling at Hijikata to commit seppuku. The shogunate is a mess and you end up rooting for Kondo's devotion to the shogunal party at the same time that you want the shogunate to dry up and blow away. The film manages to be nuanced and yet get you jumping up and down yelling at your television.
The production is beautiful, everything that you would want a samurai film to be. The costumes of the Shinsengumi are semi-accurate, unlike the beautiful but wrong ones in Gohatto. Although the pattern is correct in this film, the real colors were much more lurid. In real life, they must have looked like a troop of murderous peacocks as they charged through Kyoto. The exterior shots are gorgeous and the interior shots are lovely as well, although everybody seems to have brand new tatami at every point throughout the movie.
