This film is, quite simply, the scariest I have ever seen. As with the previous reviewer, I first saw it at the age of 11, and afterwards I couldn't sleep for days. Even now, having seen it many times, some scenes still take my breath away with sheer shock.
Its strength lies in the fact that it is not outlandishly gory or horrific, but it plays on the mind for ages before something terrifying actually happens. Even the most terrifying moments seem tame when you read about them; a ball dropping down a staircase, or a wheelchair rocking by itself sounds mundane, but when it happens in this film it will scare the hell out of you!
One thing that really sticks with me about this film - apart from the stunning performances from George C Scott and Trish van Devere - is the music. It lends so much power to the film, and it never intrudes upon the actual story; in fact, since the story is about a musician, it actually adds to the film in a unique and moving way. One tune in particular sticks in my mind, one that the main character spontaneously composes... but I won't tell any more.
A great film, sadly neglected, that deserves every star I've given it!