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The series have a strong Man vs Monster start, where both sides are likeable - you have ghouls with their cool powers, emo style, and tragic backstories, and then you have humans, who can go 1v1 with them, despite being... well, humans.
Unfortunately, the show slowly turns into a Monster vs Monster, and Man is left somewhere on the sidelines.
And while one could argue whether this made story boring or not, it's undeniable that battles took a hit. When a ghoul faced human investigator, there were some acrobatics, weapon play, and tactics, but when a ghoul fights ghoul, the fight is basically "jump on the wall -> jump on the ceiling -> poke torso with tail -> get pierced by enemy's tail -> regenerate -> repeat until one side can no longer regenerate". Moreover, as the anime progresses, ghoul powers become more and more gimmicky, until it looks like they're using magic.
The first two seasons were a solid 8, the latter two were a 6, as the story became generic, and the setting lost its charm.
So my advice would be to watch the first 2 (to get cultured), and then read summary for the rest.
