I originally gave this manga a 9, but I've changed it to 10 as of chapter 69 (nice) because the series gets better as it progresses.
One of the reasons why I skip over most shounen manga in favor of seinen is because shounen manga tends to lack real character development, or only a handful of characters develop while the rest of the cast remain static characters.
That's not the case with Jigokuraku. This manga has a large, revolving cast of criminals and Asaemons, yet the mangaka is able to add depth and development to every character, making each death impactful to both readers and other characters, as opposed to just killing off characters for the shock and gore.
Gabimaru is an outstanding and very refreshing protagonist - his staunch loyalty to his wife as his reason to survive is a welcome reprieve from the endless harems and love triangles in shounen and seinen manga. Similarly, Sagiri is a rare, well-written strong female lead - she's not crying 24/7 but she's not emotionless either, she's not pining away for the male lead like so many female leads tend to do, and she's incredibly strong and can hold her own against Gabimaru and some pretty insane enemies.
Another thing that makes this series stand out is the lack of filler action - almost every action scene (and there are a lot of them in this series) serve some kind of plot progression purpose. As a result, each release is satisfying in terms of both plot and action (as opposed to reading several chapters of back and forth fighting before getting some real dialogue), but manages to keep you on the edge of your seat, wondering what will happen to Gabimaru, Sagiri, and the gang next.
tl;dr - Jigokuraku is a must-read manga with the complexity of a seinen with well-developed characters and a fast-paced plot!