Having read 30 volumes of this series I can say without a doubt it is one of the best adventure mangas out there.
It is not a plot-focused story. It is a story of how the main character's inÂfluence grows and affects her world. It progresses based on actions and consequences rather than arbitrary plot points, and it results in character growth being much more believable than in most anime and manga. That goes not only for main characters, but also side characters who have enough depth to remain relevant long after being introduced.
That said there is a problem… Characters never feel like they are complete. There is always room for them to grow in a believable fashion, and as such there is always a path for the story to progress without feeling forced… Be prepared to invest, this story isn't ending anytime soon.