In my opinion the anime was magnificent. (9/10)
Without spoiling i can tell you that the plot is very interesting.Its plot is something similar to Berserk's eclipse but in a good way for the main characters. Undoubtedly if you don't like the gore genre its not for you.
There is a bit of romance which continues in the other ovas.
It was published more than 30 years ago so the art/sound isn't that good but for its time it was normal.
While watching it you will feel like you are in the 80-s or you are watching Michael Jackson's music video.
There is big character development in this ova and continuous in the other 2 ovas.
If you are fan of manly animes that's probably for you.
Furthermore if you are having a bad day it will surely make it better.
It's a horror anime that will probably scare the sh*t out of you but you won't know when you have watched all of the parts.And if you still don't know the order to watch all the ovas that's the way:
1. Devilman: The Birth
2. Devilman: The Demon Bird
3. Amon: The Apocalypse of Devilman