Sadly, the "Damn. Ninjas," line from the ads did not appear in the movie itself. That aside, though, I enjoyed it for what it was: stylized swordplay soap opera. It follows the same basic template as last year's Ninja Assassin, with the world's greatest killer deciding he had to break away from his assassin clan and getting hunted across the world by his clan before clashing with his surrogate father figure. Meanwhile, oriental assassins clash with gunmen and there's a lot of blood. The main difference is that the setting is a "busted town full of broken people" in the late 1800s American West, and the visuals are stylized to the point of being allegorical. It does get a bit busy, especially with the three-way battle at the end between assassin clan members, corrupt U.S. soldiers and a traveling circus. It's just jam-packed with archetypes from both spaghetti westerns and Shaw Brothers chop socky, and a lot of the story gets told in shorthand (the protagonist's life story is sketched out in maybe a minute's worth of flashbacks, but "unpacks" to a pretty full tale if you know the conventions). But it also brings in some bits of non-standard weirdness here and there, like the combat courtship scene on Christmas Eve with Popeye music (well, a classical piece that most of us heard first in Popeye cartoons) as the background.
If you're thinking of seeing it, the R rating is mainly due to the blood aerosols and several scenes of attempted rape (along with the implication that the rapist in question was usually a lot more successful).