From the earliest Melies films, The Phantom of the Opera , to The Black Swan, the ballet girls facing monsters and demons (either inner or external) are always a nice formula. Il mostro dell' opera doesn't have much to add to the pile, but yet there are some intriguing moments which set it apart from the rest of the bunch.
Most importantly, we really do have a cast full of proper dancers. Of these, Milena Vukotic really was an accomplished ballerina. The leading lady in the double role of modern-day Giulia and mediaeval Laura, Barbara Haward, is a very pretty, sensitive actress and evidently another classically trained ballerina - and yet she seems to have disappeared from the face of the earth after that movie. Anyone knows ANYTHING about her?
The musical numbers are not merely showstoppers, but actually supposed to be stemming from the narrative, i.e. like in a 1930s musical, where random people begin to sing and dance and just happen to know the intricate choreography and the choral arrangements. To make the stage cleaning process more fun, they burst into a Charleston; to keep the (utterly bloodless and sexless) vampire at bay, they decide to 'dance the devil away', even though it's never explained why frenzied rolling around to big band bebop should help one in this department.
A massive lesbian group menage and a black pussycat scaring the living daylights out of some thirty grown-up men and women. Definitely not a wasted 1,5 hours, so have your reefers rolled up.