This question is prompted by me watching the Gomorra TV series, where (I assume) most of the characters speak Neapolitan. I was curious on how intelligible the dialect (as spoken by the characters there) is to speakers of a more 'standard' Italian. Or, phrased differently: would someone from a different region of Italy need subtitles to understand what the characters were saying?
I'm from Bergamo (Lombardy) and I find Neapolitan one of the hardest dialects to understand. If I listen carefully I can get the meaning of most phrases (or at least I can deduce it), but I can't understand many of the words. Without subtitles, I can definetely understand Spanish better than Neapolitan.
I tried to watch this trailer and, even if I understand some sentences here and there, I would hate to watch this without subtitles (I grew up in the north east of Italy, near Venice).
I have problems even watching the Sicilian scenes in the Godfather without subtitles (it's a different regional language, but both are really far from my own).
I would say that barring good familiarity with the Neapolitan dialect, understanding the TV show Gomorra is very hard even for native speakers.