Some other reviewers have pointed out the similarities between this film, by envelope-pushing (and occasionally stomach-churning) director Takashi Miike, and legendary Kurosawa's "Seven Samurai". Indeed, both films tell the story of a group of honourable warriors who take up their swords against an evil force. That being said, there are a few differences.
In "Seven Samurai", the titular characters defend a helpless village against a band of marauding bandits (clearly criminals by anyone's standards). In "13 Assassins", a crack team of swordsmen is assembled to take down the Shogun's sadistic half-brother, whose continued existence threatens to destabilize the hard-earned peace of the nation (although he's not, officially, a criminal). Where "Seven Samurai" elaborates a bit more on the motivations of the protagonists, as well as how they earn the trust of the villagers they agree to protect, "13 Assassins" instead focuses on how the team is built up and how they justify dishonouring themselves by daring to challenge the authority of the Shogun's half-brother, who is a powerful, albeit reprehensible, man commanding of respect and deference.
Then there's the unrelenting, all-out fracas and blood-soaked climax and conclusion of "13 Assassins", which is in contrast to the bit-by-bit, guerilla-warfare style battle fought by the "Seven Samurai".
Has this story been told before? In a general way, perhaps. Nonetheless, Miike lends his uncompromising style to it, but thankfully not to the extent of his more cringe-worthy films. I have to say, the sword-fighting isn't spectacular. The acting is convincing, though, and the plot, themes, and messages don't go over one's head. The ensemble of characters is charming as well, from the leader of the samurai with his sheer willpower and unwavering courage, to his ne'er-do-well nephew who decides to join when womanizing and gambling fail to bring the satisfaction they once did, to the outsider from the hill country who spurns the stuffiness of samurai and prefers to use a sling and a coconut instead of a katana.
There's no such thing as a perfect movie, but "13 Assassins" has all the elements of a superlative film. It suffers only a little bit from being a tad too long and drawn-out in places. Well worth the rental or purchase.