I wont go againt the current opinion, the manga is trash compare to the anime or novel. If you've heard that Shinsekai yori is a great masterpiece and were looking to see it, it's not the good version go look for the anime or novel (both are great). However if you've allready seen one of the other 2 versions and like it, then you should consider trying the manga. There's a lot of flaw the more apparent being the yuri (sometime loli) echhi, sexuality is a one of the theme of the story indeed but while the other version did a good job at suggesting sex scene the manga just show them.
But the manga also has some good part, unfortunatly most of them are in the second half of the story, to give some exemple (attention spoiler) squealer using Human has meat shield, some fauna we didn't see before, human from other distric, and what i consider to be the best reason to read the manga we get to see the influence of a karma demon during a battle, which is really well done scene.
To conclude if you like the other version give the manga a try, because the second half has some good idea and devlopement for the world.