The manga is very good, compact, fast paced, with a good play on tension, keeping the reader on edge for the possible twists and turns of the story, that often caught me by surprise. The art is extremely detailed and probably the strongest point of the manga; it's strong western influence and the murky colored pages also contribute greatly to setting the atmosphere. The whole manga looks a lot like classic western comics and the characters' behaviours feel mostly inspired from mafia movies, with some coming straight out of western in the latter half (Indians and Ned felt straight out of Tex). Some elements of the art do draw from other manga culture, but this really is, by all means, a western comic written by Japanese author. There isn't the time to develop much the characters other than the two protagonists, who still manage to give enough emotion themselves for the story to be impactful. The manga is technically amazing, but lacks that special something for me to give it a 9.
I really appreciated some details that contribute to the immersiveness. For instance the reelaboration of historical events like the 1857 Dead Rabbits riot, which I suggest you don't read about if you don't want to spoil the story. Also Brad's gun-blade, which is the .54 caliber Elgin pistol, equipped with an 11.5 inch bowie knife blade, adopted by the US navy in 1938 and used in some instances during the US Civil War, perfect weapon for this setting.
People talk about an axe: honestly doesn't feel like there was any.