Man, I always have so many problems with this movie. To start with the obvious positive things, it is deservedly called one of the best directed horror films of all time, with a set design and style unlike any other film (even among Argento's oeuvre). Also, the music score is unbelievably eerie and effective, I'd wager that it's the best horror score of the 70s, right above Dawn Of The Dead (unsurprisingly, both films were scored by Argento's band The Goblins).
Unfortunately, the film really isn't that scary at all. It manages one or two short sequences of suspense (the blind man's death and the women behind the sheet come to mind) but for the most part Argento builds up a lot of great atmosphere, but has no real payoff to anything. And oh GOD, the performances! Outside of the two main villains, there isn't a single believable actor in the entire film.
Nevertheless, there really hasn't been another film made like this before or since. It's bright and colorful while being twisted and perverse, and the film's utterly baffling plot (which most people lament) actually improves the experience for me. Though most scenes ended with me throwing my hands in the air in utter confusion, right of the gate this quality actually begins to lull you into a dreamy half-conscious state, which is really where you need to be to fully appreciate a film like this.