I don't know why this manga is so underrated! Maybe because its anime hasn't got same story as manga has. For those who has doubt about reading this i suggest check it out, the real story will begin after 25 chapters. No spoiler at all, feel free to read this post.

Story 10/10:

Pandora heart is a rather complicated yet easy to understand story which has one of the most shining ending ever. Story begin with "Your sin is... your very existence." which somehow at the first doesn't make any sense but it will be more clear as story progress. A tragic event was happened long ago, and its detail is being revealed by every flash back, however it doesn't mean that all of story is about past. To put it in the simple way, PH is like a puzzle that its pieces place in the right position with every chapter. I felt sadness, happiness and so on with plot. Don't let that first light chapters deceive you. It will be changed to something very dark soon.

Character 9/10:

Well at the first glance characters are strange. Some character like break or will of abyss seems exotic. But when their past is manifested, their behavior will make more sense. I don't want to compare PH to Mirai Nikki, though Mirai Nikki has got many strange character, aside from main characters their behavior don't have any good explanation. For example Reisuke Houjou or Akise, Aru and so on. PH doesn't fail to make sense from none sense!
More importantly, characters are gray. there is no absolute good or evil in PH.

Vessalius, Oz: Main character. His behavior changes as he understands himself more. At the first sight he seems a typical duplicated protagonist. He is not! If you don't believe me, just read story to the end.

Nightray, Gilbert: A loyal servant or friend? He is an usual character. Nothing strange about him! He has got a well written past with Nightray, Vincent.

Alice: The master. She is a weird character with a lot of odd behavior. She makes Gilbert angry by disturbing OZ. Her past is a key to the story.

Break, Xerxes: Clown. He is one of the most astounding character in PH story. I bet he likes sugar more than his life. He assists OZ to comes to term with himself.

Vessalius, Jack: Multiple personality. An other unique character in PH (well i don't understand him, even by knowing his past.)

Overall and Enjoyment 10/10:
After this manga i read black butler and d.gray man in this genre. In my opinion they are all good, but PH is something else.