Okay so I've watched all 3 seasons and here's my thought on each of them.
SEASON 1: I really liked this season since I have read the early chapters of the manga, and it looked like a beautifully animated version of every page of that. perfect recreation! Stories and plots can be confusing and seem incomplete, but that's because a lot will be explained later on! I didn't like every single episode of this season but overall it was really enjoyable!
SEASON 2: the style of the animation and characters are a lot like , but some of the stories really REALLY don't make sense and I didn't like that.This season has basically nothing to do with the manga except some episodes. There are new characters there that are not even mentioned in the manga and the whole series is about them! And they weren't really charismatic like Sebastian and Ciel. So I didn't like to watch them. But I didn't hate the season overall because it did give some insight of the existing characters that I found really really interesting!
SEASON 3: well this shouldn't really be called "season 3" since it has a totally different mood and animation style and you'll notice it the moment you start watching the first episode. It's called "The Book of Circus" following this 10 episode season, there are 2 episode called "the Book of Murder" and after that is the movie, "The Book of Atlantic". These three "the book of.." are all tied together and totally different from the Black Butler season 1 and 2. So don't expect to get the same feels you got in the previous seasons for these as well. But I LOVE them! They're too much like the manga and they make a lot more sense! I do believe the Book of Circus should've been a lot and I mean A LOT shorter thought. 10 episodes is too much for it and it gets boring by the end of it. I hated it the first time I watched it but then as I was searching in the internet I found out there are some hidden notes in all the flash backs that are showed there that are really really important to what the manga is getting right now. I think they WANTED to make sure everything in the manga is in the anime for this sake. but overall, it's possible to hate "season 3". but if you're a fan girl like me, and you know you WILL go through the internet just to understand every bit of the anime, you're gonna love it.
SEASON 2: the style of the animation and characters are a lot like , but some of the stories really REALLY don't make sense and I didn't like that.This season has basically nothing to do with the manga except some episodes. There are new characters there that are not even mentioned in the manga and the whole series is about them! And they weren't really charismatic like Sebastian and Ciel. So I didn't like to watch them. But I didn't hate the season overall because it did give some insight of the existing characters that I found really really interesting!
SEASON 3: well this shouldn't really be called "season 3" since it has a totally different mood and animation style and you'll notice it the moment you start watching the first episode. It's called "The Book of Circus" following this 10 episode season, there are 2 episode called "the Book of Murder" and after that is the movie, "The Book of Atlantic". These three "the book of.." are all tied together and totally different from the Black Butler season 1 and 2. So don't expect to get the same feels you got in the previous seasons for these as well. But I LOVE them! They're too much like the manga and they make a lot more sense! I do believe the Book of Circus should've been a lot and I mean A LOT shorter thought. 10 episodes is too much for it and it gets boring by the end of it. I hated it the first time I watched it but then as I was searching in the internet I found out there are some hidden notes in all the flash backs that are showed there that are really really important to what the manga is getting right now. I think they WANTED to make sure everything in the manga is in the anime for this sake. but overall, it's possible to hate "season 3". but if you're a fan girl like me, and you know you WILL go through the internet just to understand every bit of the anime, you're gonna love it.