A classic, absurd, hyperviolent manga with cool action, interesting ideas, and great art.... SOMETIMES. I love Riki-Oh, but it wavers a lot in its quality. It'll peak here and there, then dip down into being something just barely mediocre. Obvious peaks are in it's first arc through to the beginning of the cape (I really like the post prison bits with Chou Zenki and the fortuneteller, but it gets a bit uninteresting once the Black Knights show up and the plot of the second arc starts to come up), the fight with The Baron, and the final arc before and after all of the overly religious stuff.
Riki-Oh is a story filled with great potential, but more often than not, it comes up short. I am a big fan of the story and a decent bit of the characters, but I wouldn't recommend this to just anybody. Most of the guys I've referred it to enjoyed it, but it all depends on your tastes. Lots of characters and plot points are dropped, there's a lot of copy and pasting of panels due to Tetsuya's laziness, I think that Riki-Oh could really profit off of a well thought out anime remake.
If you came here after loving the movie, read it, but don't hesitate to drop it if you're starting to feel cold on the story. Nobody will fault you for not enjoying it's crazy story. If you're a fan of the standard psychoviolence of Saruwatari, and have somehow missed reading this, jump on in. And if you've finished the story, but haven't watched the live action movie, WATCH THE DAMN MOVIE!
The best fights would have to be Riki Vs The Warden, Riki Vs The Baron, and Riki Vs Mukai. Honorable mention to Riki Vs Yomi from the movie... Great choreography, and filled in spots that the manga otherwise left out.