Deadman wonderland, while not perfect, is a phenomenal manga. While I adore the show, the manga ultimately outshines it in quality and quantity, tying up the story and telling stories the anime never got to. If you enjoyed the show, then the manga will go beyond that.
It's worth noting that this series is not for everyone. If you dislike gore and the dark, gory genre of manga, this might not be your cup of tea.
Story: 8/10
In my opinion, the weakest point of the manga. At first, the whole thing feels rather cookie-cutter with the death games, but it does become better as it goes on (as I think is true with most things in this series).
Art:10/10
The art from this manga is something to return to over and over again. The way each branch of sin looks is unique and interesting, and many individual panels of a fight are absolutely awesome. The art during such fights is phenomenal, and something that really makes this series stand out. When there is no action happening, the artists play around with the character's expressions to give them a lot of personality. All of the character designs, while some (shiro) are initially jarring, are all very good and fit with the character.
Characters: 9/10
Ganta gets a lot of flak, but at the end of the day, he's a yound kid thrown into a nightmare scenario. What makes him special is his dedication to his friends and refusal to give up, no matter what. It's these traits that make him someone you can root for. The manga's continued story really helps allow him to grow as a character, because this manga gets better as it goes on. This category is where the show missed out a lot: in the manga, we get the awesome characters Chaplin, Idaki, and Azami, who never got to appear in the show. Chaplin in particular is worth reading for, as she's one of the best-written and most like-able transgender manga characters. All of the characters are flawed and broke, and it's because of this that they all are so great - nobody's perfect, and deadman wonderland knows this. Despite this, you can still sympathise with and love all of them, despite their flaws (or maybe because of them).
Enjoyment: 10/10
This is, of course, subjective, but I loved this manga. I had never read a manga sereis before (just watched anime), but the show's incomplete story led me to try and read it. Ultimately, the manga far outshines the show in almost every category, and is well worth your time.
Overall: 9/10
Not perfect, but a damn good manga and something that needs more attention.