No doubt, this movie is very bad, much worse than predecessor, even worse than the worst comic-book episode. Plot is naïve with lot of illogical points. One has to invest a lot of suspension of disbelief if wants to endure it. But... If you do not take it seriously, you can relax and enjoy it. So bad that it is good, great guilty pleasure, genuine camp. By the way, reconstruction of 1960s is nice, with great accessories and, of course, Italian cars and Italian cities (with my favorite Trieste).
Ms. Bellucci's acting was so bad that I think she was self-ironical and intentionally making fun of the movie. I prefer Mr. Marinelli from the first movie as Diabolik. Better actor with charisma and better looks for main character. Mr. Gianniotti luckily does not have a lot of screen time what is kind of ironic too. Ms. Leone is gorgeous as always.