I liked the premise of it. The idea of a timid, good-hearted guy gaining the strength and bloodthirstiness of a demon sounded really fascinating to me. It was what drew me in initially and drew me in for the rest of the series. What's interesting is that it isn't a split personality deal where two separate personalities are in conflict with each other, nor is it a Bruce Banner/Hulk situation where Akira remains normal as a human but changes entirely when taking on his full demon form. Both the traits that make up Akira and Amon are formed as one whole being, and as such, Akira never really takes notice to his drastic change in personality. Or rather, he does, but (thanks to the demon personality) he accepts the change quite eagerly.
Though it appears that the demon Amon's personality dominates Akira, the story makes it clear that it's Akira who still retains ultimate control over himself and the demon he's merged with, beginning and ending with his utter determination to save the human race. In the end, his humanity hinges on his love for Miki, and no better is this shown than in their interactions together; I loved how whenever Miki tried to flirt or show affection towards Akira in his now-merged state, he'd revert back to his old flustered and shy self.
Speaking of Miki, I really dug her. She was cute and funny, but also foulmouthed and tough as nails. As mentioned above, I loved that, even after Akira got his demon powers, she was still the one who clearly wore the pants in the relationship. With all of that said, it was heartwrenching to see her get killed the way she did. At least she went down fighting.
The last couple of volumes were just a downward spiral of madness. With humanity eventually ripping itself apart along with Akira losing his faith in it, Nagai really hammers his antiwar message home and then some. Some may find it too much or too unsubtle, and I won't argue against it, but I found it wholly appropriate for the given setting and Nagai's intended message. Reading this interview prior to reading the series also really helped in putting it into perspective.
I figured out the Ryo=Satan twist a little whiles before the reveal, but it was still shocking all the same. Not sure what to make of the "I fell in love with Akira because I'm a hermaphrodite!", though...
So where was the "potential it didn't quite meet" I was referring to earlier? Mostly, I wished the cast got some fleshing out before shit went down. Or maybe not so much fleshing out as I wished to see more of their interaction in a normal setting free of demon peril. More quieter moments, basically. Granted, it did help the story to progress faster and make the "And then they all died" ending seem like less of a waste, but I also think it would have made the sense of impending doom all the more intense and put even more emphasis on just why War Is Bad.
Still, when it comes down to it, I mostly loved this series, enough that I'd like to own it. I know I'm going to check out the OVAs, and I'm even thinking of giving the NES game a try since I heard an English patch has been made for it. Plus it has the option of a happy ending.