Interesting but badly flawed. This is not one of Argento's best by any means, but it's better than Phantom of the Opera and does have some things going for it. The first sleepwalking scene is excellent, with exhilarating subjective camerawork and weird lighting effects. About the last fifteen minutes is superb and pretty intense as it builds, with two genuine shocks at the end after some increasingly weird grotesqueness. I found the very end to be quite haunting.
The problems, however, are numerous. Argento can't direct actors, and here usually good performers give wildly erratic displays. Donald Pleasence's Scottish accent is awful and serves no purpose except to presumably make him a cuddly figure. Jennifer Connelly often looks sedated, and this is the worst performance of her career. In their defence, they do have to contend with some terrible dialogue. Plot and dialogue have never been Argento's strong suit, but here he is far worse than usual.
The theme music is eerie and effective but for no apparent reason is occasionally interrupted by thunderous and indescribably bad mid-80s heavy metal. Sometimes the use of music seems totally random. The scene with the phone would have been much more tense without the blaring music drowning out the suspense. In Suspiria he showed total mastery of using music as an aid to suspense, but here you get the feeling he just wants to thrust his current favourite records on the viewer.
Rather than taut and controlled like in Tenebrae or Suspiria, here the plot is loose, nonsensical and it sags badly at times, limping along somewhat till near the end when Argento wakes up and kicks things up a gear, rescuing the entire film.
Generally Argento's films can easily be divided into two camps: his giallo murder mysteries and his more Gothic supernatural films. This seems to fall between the two, without really being a particularly good example of either: a murder mystery that is uninvolving and doesn't make sense, and surreal horror that is often a bit clumsy. It isn't a bad film it just doesn't hold a candle to Inferno etc.
I would recommend this to Argento fans. It is worth seeing and has interesting elements, but it is deeply flawed and not one of his best.