Let me start off by saying that this manga is pure fanservice and I'm talking about both kinds here. If you are neither a fan of the original Devilman manga nor of fetishized sexual violence (more on that later), you probably won't find much to like about Devilman G.
The story is a re-imagining of the original Devilman manga that puts Miki Makimura into main character position alongside Akira and incorporates aspects of the Toei anime adaptation and Go Nagai's other works. The story is fine, it's unpredictable and does its own thing while still being "Devilman," but it was the art that kept me turning
The art is simply amazing and a real feast for the eyes. Rui Takato has modernized Go Nagai's style beautifully and fans of the original Devilman manga will get a kick out of how "Go Nagai" some of the characters look and the way certain scenes are depicted. My only gripe is that Miki is drawn in a more contemporary "moe" style and sometimes seems at odds with the classic look of the other characters, though that is not to say she looks bad at all, just out of place in some scenes.
Now, if the words "sexual violence" make you feel any kind of negative emotion at all, run far away from Devilman G. Clothes are constantly getting torn off, there's lots of rape and torture, naked girls getting dismembered or sliced in half, and someone gets impaled through the vagina all the way up and out through her chest. And it's all shockingly explicit for a real manga. I can't believe this was published in the same mag as Princess Tutu, or even published at all. If you're a weird sicko like me (or Go Nagai) you'll enjoy every page, but Devilman G is certainly made for a specific audience and those outside that audience are going to be grossed out, so be warned.