personally, i love this manga, and i think it really worths spending time to read
it's totally not mainstream at all, unpredictable plot twists, the battles ... well not as flashy as the other shounen mangas but still breath-taking. And yes, the plot has a lot of things that we have to brainstorm
the characters are all great and unique, especially the main characters - Junya and Ando, they have great character growth which i really enjoy.
Someone below says "they're just there to make the story flow. On the contrary, the assassins/hit-men are more of the type that appeal to me, and even the ones with
no special powers are still awesome." => OK, i shall laugh of pity cuz IT'S Junya and Ando who make this manga awesome. It appears that you mainly judge characters from the outside "cool, badass blah blah ..."