This movie felt like a very long episode of a mini series from Italian tv; the acting was not very natural and made me chuckle/cringe lots of times. It doesn't help that they didn't change the dialogues much from the original comics, which are from the 60s, so this can be a good thing or bad thing depending on a lot of factors.
However I do think this is the best movie Diabolik fans are gonna get: it's very loyal to the comics and has nice photography, soundtrack (which reminded me a little of Danny Elfman's score for Burton's Batman, especially in the first part of the movie) and special effects.
Of course it has some faults, and unfortunately one of these is pretty big. I know that people likes Miriam Leone, JESUS, I KNOW. But let's face the fact: she is not a very good actress. She did look like a young Eva Kant, but dear God, her acting level is very basic and soap opera level. Luca Marinelli wasn't very good either and definitely not my kind of Diabolik. I don't know whose idea was to make him whisper in a monotone voice instead of talking normally. He was trying so hard to be this serious, confident chracter, but it just didn't work for me... But again, you work with what you have, and Diabolik has such distinctive traits (to name a few: shiny black hair with widow peak, steel-colored eyes with long lashes) that I can't think of an Italian actor who could do the character justice.
Valerio Mastandrea as Ginko was nice, but not exceptional.
Intro of the movie was pretty cool, with Manuel Agnelli's new single "La profondità degli abissi"; however the rest of the film dragged on a little too long, thanks to the expository dialogues.

In conclusion, I appreciate the effort and as fan of Diabolik I would have probably enjoyed this as a Netflix mini-series more than a long-ass movie. Still worth a watch.