While I gather from outside sources that the OVA diverges from the manga, I can otherwise claim no knowledge of the source material. If nothing else, this puts me in a prime position to judge this anime on its own merits. The core foci here are readily apparent from the get-go: gnarly, even grotesque, and imaginative creature designs, accordingly credited to Nirasawa Yasushi; extremely strong, graphic violence, some of it very sexual in nature, and substantial blood and gore; and foremost themes of chaos in a broken society, amidst a war against otherworldly monsters, as desperate, mistrusting humans turn against each other and seem just as awful as the demons whose loyalties are similarly flimsy. These facets so heavily represent the intended and perceived value of 'Amon: Apocalypse of Devilman' that it's not unreasonable to say that they are the primary reasons one might have to watch in the first place. That's not to say that this isn't also worthwhile in other capacities, but the strengths here - the aspects that received the most attention - are plainly evident.
Now, as to what else the feature may have to offer: first of all, Hayasaka Ritsuko's loose adaptation of (part of) Nagai Go's manga does give us a story - sort of, kind of, more or less - but it's frankly nothing more than a vehicle for the creature designs and the violence. If you're looking for something of substance, move along and keep looking; the plot is baseline adequate, but very small and very thin, and doesn't really illuminate any of the ideas that presumably fill the manga. Someone like me who watches with no knowledge of the manga has no idea what is supposed to be going on beyond a few pointlessly broad plot points. (Demons, Humans, and Other? Check. Some seeming act of betrayal? Check.) Some of the dialogue is just plain bad; slight and single-minded as the plot is, much the same could be said of the scene writing, with part of a fight near the end pointedly raising a skeptical eyebrow. On a like note, the voice acting ably serves its purpose, but it's a means to an end more than anything else; the original music of Mangahead and Kobayashi Takeshi is suitable moody complement for the proceedings, but doesn't particularly stand out.
And, well, that just leaves the animation, doesn't it? As someone who has only very recently begun to remotely dip their toes into anime, I earnestly believe that far more than not artists of the Japanese industry handily outpace their counterparts elsewhere around the world. We routinely see detail, texture, and shrewd considerations in 2D Japanese animation that we don't from other regions; forget 3D animation altogether, except where sparsely and intently employed. I don't know if I'd necessarily say that the doing here is the top of the line - there are distinct weak spots, like short scenes of characters running - but even at that it's safe to say that 'Amon: Apocalypse of Devilman' easily fits well within that same general category. Detail, texture, lighting and shading, color, backgrounds and environments, interiors and exteriors, humans and creatures, almost nauseatingly violent but fluid action sequences, effects and other active elements: in most every regard the visual experience is simply fantastic. If all one wants out of this OVA is the animation, one should be quite satisfied. And with the underlying thrust simply being those creatures, that violence, and the depiction of Societal Chaos, after all, the animation is the most important part.
The question is: Is this enough? The flick had very simple goals, and it achieved them, so I suppose that's a job well done. I can't say it isn't darkly entertaining, such as it is. However, I could have done with more effort - any effort - to actually tell a story, because that just doesn't happen here. 'Amon' keeps a couple uselessly non-specific notions in mind, then just charges full-steam ahead with its visuals. All told I guess the sum total is enjoyable, but it's not at all fulfilling; this is a small appetizer, not a full portion, let alone an entree. That's all it wanted to be, and maybe that's all it needed to be, so 'Apocalypse of Devilman' IS "good," in one measure or another. But unless you're a huge, diehard fan, or have some very special impetus to watch, there is no need at all to seek this out. If you happen to come across it, save it for a quick watch on a lazy night, and that may be the best way to appreciate this OVA.