Started watching this on Netflix. The first episode definitely is a great attention grabber providing a good peek into the protagonist and the story for the first season.
I also liked how even though the main plotline changes (and introduces different antagonists)/between the different seasons the link to the main character remains continuous in a very fluid fashion.
My main criticism is that several episodes in the final seasons do get slower and a bit overdramatic as compared to the pace of the first few seasons. Also some key characters in the final season seem to be specifically created and introduced to plug some holes in the plot.
Despite the criticism, I would definitely recommend this show to folks who have watched and enjoyed anime like demon hunter, kabaneri of iron fortress, re-zero and similarly themed anime.