While Yojimbo was a complex film about a masterless samurai playing two gang factions against each other, and was, at times, a bit clunky because of it, Sanjuro is a much simpler film.* The result is that it is simply not up to the standard that the first film set, which isn't such a strange thing when considering the long history of sequels that didn't equal the original, but the simplicity of Sanjuro is a bit disappointing. The plot works on a town that has corruption in it, and a group of young officials wish to stamp it out, going first to one official, who tells them they're best to forget it, and then to a second, who in fact is the leader of the corruption, and thus organises their death. Except, of course, that Toshiro Mifune's nameless (or name changing) samurai is there to help them.
Just like that. No hint of doing it for money, of getting something out of it himself... nothing. He's just there, sleeping, overhears, and helps out.
Ultimately, the character motivation is what is missing from Sanjuro, which is a shame, because it's not a bad film, and at the centre of it is Mifune's excellent portrayal of a ragged, down and out samurai. There's just a whole heap of charm in him and his rough and tumble, sake drinking exterior, that it's possible to switch your brain off on the minor plot, and enjoy his interactions with the nine young men, which is always quite funny. One of my favourite scenes in the film is when Mifune leads the nine men on a rescue, and they follow him so closely that, as he notes, they look just like a centipede.
As with all samurai films, Sanjuro features its collection of sword fighting, which is handled smoothly throughout the film, culminating in a very fine duel at the end.
However, in the end, Sanjuro is just a minor Kurosawa film. Enjoyable, funny in places, and with the always excellent Toshiro Mifune, but still, minor.
*What A Fistful of Dollars did at the very least, was smooth out Yojimbo's narrative flow. In addition, Eastwood's Man With No Name (or, the man whose name is changed in each film, much like in Yojimbo and Sanjuro, though not as a plot point) is a much more reserved, darker character. I've no idea if Eastwood saw Yojimbo but the changes he made to the character were utterly necessary to giving A Fistful of Dollars its own voice.
As an interesting aside, people watching Sanjuro will find a scene that Lucas lifted for Star Wars. It's only a tiny one, and not to the extent that, by all accounts, the Hidden Fortress was an influence, but it's there. In addition, Lucas apparently considered having Mifune play Obi Won Kenobi, which, while i love Alec Guiness, would have had it's own charm to it, i think.