It peaks at the beginning, unfortunately. Ep.1 is great, while eps.2-3 are okay. I still enjoyed it, though. It's more interesting than you'd expect from the title.
CYBORG 009 (1964, Shotaro Ishinomori but his work looks like Osamu Tezuka's) = nine people who were kidnapped and turned into cyborgs by an evil organisation, Black Ghost. They've got superpowers. They escaped and they've been fighting Black Ghost ever since.
Cyborg 009's ending had been planned as an arc called "Battle of the Gods", which fits with...
DEVILMAN (1972, Go Nagai) = Akira Fudou has a cute not-sister (Miki), a loyal best friend (Ryou) and a secret life as the devil. That's not metaphorical. He's literally a devil, with horns and wings, although he likes to insist that he still has a human heart. On meeting other monsters like himself, Akira will transform into Devilman and perpetrate ultra-violence, usually with gore and severed limbs.
Devilman's ending is the Book of Revelation, but darker.
That religious link makes these apparently mismatching franchises fit better than you'd think. They're also similar in having old-fashioned art styles that look far more interesting and wacky than modern anime. I'd have liked to see that embraced more wholeheartedly, but these 2015 OVAs are going a respectful halfway towards them. Devilman's humans don't look Nagai-ish enough, but the devils are everything you could hope for and the cyborgs' Tezuka-ish designs will make you squee. You could hit a home run with Jet Link's hair, while there's a comedy nose competition going on. (Dr. Adams's character design has unfortunately been modernised, though.)
The strongest sequences are the start and end of ep.1. The mini-series starts by retelling a selected highlight of each franchise's original manga. Cyborg 009 fights the god Apollo. "When you gain powers beyond a human, you become a god or a devil. I became a god." Taking the other path, of course, was Devilman. We see his fight with Jinmen, which is oddly more faithful to the manga than the 1990 OVA version.
Both heroes lose a sister. Devilman's role in this is more active.
The franchises learn about each other, as demons are sighted and people go missing. A rave becomes a Sabbat. Ultimately, Cyborg 009 himself goes toe to toe with Devilman, in a fight we'd been waiting all episode to see. That was cool. (And also limb-shatteringly violent.)
After that, though, things calm down. The cyborgs realise that Devilman's not the problem. Akira mostly sits out ep.2. A cyborg called Eva goes to school and looks for Miki. There are plenty of fights, but fortunately that's not a minus. They're exciting. We know the combatants, they've got superpowers and there's always emotional context.
Ep.3 provides a decent ending. It's not just a big fight, with the most important person being Eva and her desire to save her brother. (If this means a nude sibling embrace... well, welcome to anime.) The episode trumps its own cybernetic demon, while Cyborg 009 gets a lesson from Devilman in what it means to be human. (Akira's always known what he needs to keep alive within himself, but Cyborg 009 had sort of forgotten that.)
Overall, this story goes from "great" to "okay". It works. It convinces you that its apparently mismatching elements fit. It's faithful to the originals and taking them seriously. It's also directed by Jun Kawagoe, who'd previously worked on the 2001 Cyborg 009 anime. It has a slight failure of nerve with the character designs, but I'm happy with what we did get.
You know, I've just realised that I never wrote a proper review for Mazinger Z vs. Devilman...