Four Flies on Grey Velvet has one of the worst protagonists I've seen in a movie in a good while.
It's also a little boring, when it comes to pacing and what it's actually about, though there are a few creative sequences spread thinly throughout, and the film's strange sense of humor kind of helped make it a little more interesting.
Otherwise, I feel like this'll only really do it for people who really love Dario Argento movies. I've never been the biggest fan (I definitely like a bunch), but I don't think this is up there among his better ones. As such, maybe Four Flies on Grey Velvet's recommendable to those who feel starved for classic giallo, but even then, a recommendation of that sort still couldn't be too enthusiastic.