It might not be believed, but I do not use the word 'masterpiece' lightly, but you know what? Vagabond qualifies.
It is an artistic rewrite of the story of the great warrior Miyamoto Musashi. It begins after a pivotal moment in his life (Battle of Sekigahara), but the manga also takes pain to provide a relentless amount of exposition/back story/different perspectives/etc, so one always seem to have a strong grasp of the story and characters. It is also incredibly helpful in creating very human characters who are at times repulsive and sympathetic at other times.
All in all, I feel that the manga's strength lies in the artwork (unsurprisingly) and its ability to weave an immersive, moving, and emotionally involving tale. When there is a battle, I really felt a rush of adrenaline and a sense of excitement. When there is a death, I really felt touched and, admittedly, I sometimes bawled like a baby. Some subplots in the manga really hit me hard with that and the manga definitely likes throwing tearjerkers.
The manga takes an unflinching approach to the realities the characters face. Battles are bloody and full of maimed body parts, prostitutes are plentiful and celebrated, secondary characters faces incoming terror with a mix of blatantly cowardliness, resignation, and stoicism. It can get a bit intense at times, but it works.
I speak highly of the manga and rightfully so, but it does have its weaknesses. Not all of the characters are written well (including some very important characters) making for a weaker story at times, story arcs sometimes drag on and this feels worse during the less interesting arcs or moments in the story, a tendency for the characters to sometimes get into self-important nonsensical preaches about life, and etc.
Overall, the problems are relatively minor and Vagabond is still easily among the best mangas around. All possible recommendations to give this a chance!