This anime isn't bad but watching it first you'll miss the unique opportunity to be amazed by this wonderful story. The real story is too good to miss it that way.
Whether you prefer to read the manga or the novel is up to you because they are really close. Add the anime only as a bonus _afterwards_! I'm no one who prefers a books over a movie on principle but here it does matter a lot and decides whether you're going to cry from feelings or whether you'll be very disappointed.
No.6 has an amazing story which is touching, heavy, beautiful, scaring, anxious, heart-warming with characters who develop really much. The anime misses all kinds of moments which strengthen the bound between the characters as well as those that show their duality way clearer. Everything that is happening looks rather random! It's totally not that understanding, touching and _real_ like the manga/novel. Important moments are pressed together without the real atmosphere of the places.
If you read the manga/novel it might let you be stunned for a long time. And if you watch the anime _afterwards_ you'll totally understand why I wrote this.
Furthermore, the anime's end is extremely unsatisfying.
As I already said, this anime isn't bad but it's nothing to be compared with the real story.
Even the ranking shows it:
Manga: #157
Novel: #36
Anime: #787
The manga's art is beautiful and vivid. I kind of even preferred the manga (it is completed) and read the the end of the novel + "No.6 beyond" as an extra to know what will happen after.
