Truthfully, it's no better or worse than any other late '70s thriller. It's like a Pakula film played on 3/4 speed. Willis as a director leaves out the 'exciting' parts and gives away the one twist after 20 minutes. I'd like to think it's not incompetence and just an excuse for mid-century noir style.
Film students should watch closely: Willis' beautifully composed frames are all top-notch. The film score by Ennio Morricone also fits the film like a glove. Don't know why this film was so maligned by critics when it came out.