There is too many reasons why I think so. Let's see:
- No real antagonist / protagonist (I mean, the three main characters are: a criminal born on the worst place imaginable, a ronin that killed his master and a girl who's father betrayed the country by choosing another religion)
- Vulgar language / One of the best dub's ever (We all like curses and rotten words, and here its dealt like free candy)
- Both Mugin and Jin are overpowered (except against that blind girl and at some points at the end, but other than that... hell, these guys butcher their way through a mountain of corpses)
- Stupid and awesome humor (I mean, every episode is a combination of awesome and LOOOOOL awesome. The humor is easy going and understandable but quite funny to get a laugh, a smile or even a smirk from you. Also Mugin is as simple minded as they get, so you can always expect quality bullshit humor from him)
- Dirty and Honest (Not a ton of ecchi and none is shown, but throughout the anime the whole recognition of whores, sluts and what not is openly shown and in some episodes the driving engine. The anime is rotten dirty, but not perverted in the standard ecchi anime way)
- Not a huge goal driven anime (Sure, there is the main plot but that's just the anchor that characters move towards the series. The real plot twists happen every episode and they are all different. Its basically a Slice of Life anime with every other element, especially action. You get hooked on fast and hard. Its like heroin, but free and not as damaging)
- Enter random wise sentence here
Just watch it if you haven't. 26 episodes, all of them worth it.
