I consider myself a casual fan of Dario Argento. For every really good flick by him (such as "Deep Red" and "Tenebre"), there seems to be one that's equally uninspired (his output since "Opera" has tended to disappoint). Still, there's no denying that when he's at his best, there are few horror directors who can top him. I consider "Opera" to be Argento's finest work.
This is definitely the film where all of his trademarks are present. There's no well-developed characters and the plot makes very little sense once you begin to scrutinize it. Still, think of Argento as an European equivalent of Brian DePalma - the style is all that matters here, so much it becomes the substance. Typical to Argento, there's some beautifully filmed murder sequences. Those unfamiliar with the man's work may perceive that as a sadistic remark (and it may be, considering the often time misogyny of them), but its really true. Argento stages murder with an intricacy to eye-popping visual detail. He truly makes graphic violence an art form. Plus, his direction and the look of his films is impeccable.
Fortunately, "Opera" is one of his most frightening products. Argento is the only director whom I feel booming rock music works well for horror sequences (when others attempt to pull it off, it comes across as cheesy). The acting varies, with Cristina Marsillach playing a beautiful and reasonably sympathetic but never particularly animate young opera performer. The rest of the actors are workmanlike and get the job done, but again, with Argento, the acting is never the point. Its the beautiful colors, the terrifying violence, and the overall fever dream / nightmare atmosphere. (9/10)