I both loved this anime and truly wish I didn't watch it.
This is one of the few animes that I watched in one sitting (yes, all 26 episodes). It was just so interesting and thrilling that I had to keep watching. I became really invested in the characters and the story, so I had to know what happened.
There was a ton that I really love about Mirai Nikki. I think Yuno is very cute and, when she is not a complete psycho, is a very endearing character. She is portrayed in a way that makes viewers kind of like her despite how crazy she is. I think that can be a difficult thing to do. Though you like her in some ways, you can also never ignore her faults (like oh, you know, her murderous tendancies...).
I also thought the psychological and physical battles between Yukki and Yuno and the other contestants were well done. Though disturbing, the strategy and complecity of some of the fights were really intriguing. The battle with the little kid is simutaneously my favorite and least favorite battle because of how scary and complex it was.
I also liked how the backstories of every contestant was unique and important to the story. You learned why each one fought, why each was desperate or crazy. It really gave depth to the characters and made you consider whether or not Yukki and Yuno were really the good guy, or if anyone was in the right at all.
However, there were a few thing about the series that bugged me.
Though many like Yukki because he has (sort of) realistic reactions to everything going on, I really dislike him. He is so annoying and never even tries to defend himself well. He's constantly flip-flopping and is never loyal to anyone. He doesn't grow much as a character. He learns new things, but doesn't really change himself. He's not th worst character in the world, but certainly not my favorite.
Also, this anime is not for the faint of heart. There are tons of disturbing sequences (violence, rape, etc.) and those who could be triggered by anything like that should not watch it. I found some of it too disturbing, but I understood the function of that level of twistedness in the context of the show.
Anywho, I think if you like psychological thriller animes (or I guess if you just like extreme Yanderes) this is a good one to watch.
