(NO SPOILER REVIEW)
You might have found this manga when looking into the Devilman universe and Go Nagai's works, that's how I found it. This review is based only on Maou Dante, as I haven't read any other work by Nagai. I you don't want to read this looong review, skip to the end where I give my final thoughts.

STORY: A really different story to what you might've ever seen. This is a short story, you can find various versions with different amount of volumes, but they're all (mostly) the same. I will explain why "mostly" in the art section. A controversial plot for it's time, Go Nagai portrays a different angle between the battle of "Good and Evil". While the story is interesting, certain aspects of the plot are pretty goofy, and it might be hard for you to take it as seriously as the manga portrays it. Also, like most of Nagai works (which I haven't read, but know of), this is full of gruesome deaths and blood everywhere. BE AWARE, this manga doesn't have an ending.

ART: I'm not a fan of Nagai's artstyle. It might have been okay in the 70's, but today it's really hard to like it if you don't look at it retroactively or with an open mind. The strongest point of this manga's artwork is by far, the backgrounds and the giant monsters, which are full to the brim with detail. The weakest point is,of course, the character designs and overall style. Horrible aesthetic plus a laughable anatomy, even for a product of it's time, anatomy as bad as this one shouldn't have been okay.
There are different versions of this manga by different publishers that vary between the amount of pages and volumes, but they're not different enough to be dissatisfied with if you read a "lesser" version. The 2 volume version is the original, while the 3 volume version has some colored pages, some entirely new pages it also takes away a few pages entirely, not many, maybe like 4 or 5) and fixed character designs. By "fixed" I mean that Nagai redrew some panels that looked terrible on the original, like character expressions, clothes, shading and anatomy. I read both versions, and this fixes are more than welcome.

CHARACTER: I have no positives or negatives when it comes to the characters, none of them get enough development in the span of 2 volumes to make an opinion off.

ENJOYMENT & Overall Opinion: A short and easy to read manga that presents an interesting story (Albeit, a little bit boring in my opinion) with a subpar artstyle that is only good in the "kaiju battles" and the backgrounds. Would only recommend it if you plan on reading the entire universe of Devilman mangas, considering "Maou Dante" as a proto-Devilman story.
There's a Remake of this called "NEO MAOU DANTE" with better art and an actual ending, considering this version doesn't have a proper ending at all. I recommend reading Maou Dante before going for Neo Maou Dante.