As I see it, this series has two endings. The first is the ending of the last chapter of Mirai Nikki, and the second is the "future chapter." The first ending is obviously superior, but since the second ending isn't very serious it's probably something thrown in to please the shounen readers who wanted a happy ending.
What I found really interesting in this series is the characters, and especially Yuno. This might be because I really like interesting yandere characters, and I think Yuno qualifies for this because of what we get to learn about her past and development. People who end up becoming batshit insane weren't always that way, and I think it's interesting learning about what made them that way. Yukiteru hasn't been nearly as interesting a character, but in the end we also ended up knowing more about him and his relationship with Yuno. I don't think main characters always have to be entertaining, unless you're reading something for pure entertainment, but it's important that there's something interesting to be learned from them. And in the case of Yukiteru, I think there was.
Regardless, I would normally give this manga 8/10 for its characters and original or shocking elements, even though the story could be more coherent and less formulatic at times. But I'm giving it 9/10 because I'm biased.