Personally, I can't imagine how a masterpiece-level series would ever have a poor adaption. That was before I read this manga.
Everything is wrong in this terrible adaption. The story which is the series' best aspect is turned into a pile of rubbish, by totally changing the flow of the events, which in the novel and anime are linked together incredibly well. The same-sex relationship theme which is essential in the world-building is shown here as nothing more than utterly disgusting yuri fanservice. The mangaka thought that maybe adding a little gore to the story would make it better. Little did he know that his "effort" was wasted.
The characters are fleshed out completely different, even changing their personalities drastically.
The only "redeeming" (if one can call it that) quality is the art, which at times was detailed and nice to look at.
If you were amazed by the anime of Shinsekai Yori, DO NOT try the manga version. Instead, if you want additional details, read the novel, which is currently being translated online and has reached the final arc.
