Set at an ever increasing, feverish pace, Devilman is a flawed masterpiece. I find this incredibly hard to rate, because it's simply all over the place. It's easily finished within three or four hours, it feels like a two films, one set in a reasonable setting, and the second in a dream.
The framework for universe is great. There are rules, they make enough sense to be believable, and there's an attempt to adhere to these rules. Simple enough. It mixes in a few genres, and what it attempts in the early chapters works. There's the familiar shounen school setting, a timid boy given powers who becomes more confident (though in Devilman's case, simply becomes rad over night), and he's given a clear goal to work with. Mystery is woven in, along with horror and the very occasional bit of humor, and I was hooked.
The art is great and only improves throughout, coupled with some fantastic panel flow that gives a pretty visceral sense of the action. There's some no holds barred frames scattered throughout the entire series that stick with me, along with one of the creepiest images I've ever seen in a horror manga. There's nothing to complain about here. It works, and it works well.
What I find the most interesting about Devilman is there's not a single part that doesn't have issues, yet it somehow works despite its parts. For instance, the characters almost don't matter. They're pretty stereotypical, and with only five volumes you never truly get a sense of who they are. Go Nagai didn't take time with this story, never really giving it a pace that makes sense. The first couple of chapters happen sequentially, but after that, time can be skipped in huge chunks with only a small bit of exposition later to explain how long it has been, if any exposition is given at all. Many times, details are dropped and seemingly forgotten, without any explanation that I'm aware of. Still, I don't care, because the cohesion is great.
The ending, which takes up pretty much the last two novels, is a whirlwind. I don't want to give too much away, so I'll be vague, but the tone and scope shift so much, it basically becomes another genre. Before I read Devilman or anything related to it, I'd already heard about the ending, which it's quite famous for, and understandably so.
Devilman simply knows what it is. Go Nagai had some concepts in mind, enough to string a narrative along to reach an end goal, and he did just that. The story has very few frills. It doesn't slow down to explore anything beyond the main point, and that is a bit sad to me. I would have liked to have seen more from the world he was building, for him to take some time to attach us to the characters and give it another edge to the punches he knows how to throw. Still, despite it's questionable pacing and character issues, it's easy to see the genius at work here, and ultimately, Devilman is a ballsy, weird, and fun ride.