There are some very good features to this thriller that make up for its occasional flaws. Joan Crawford is very good in a role that gives her a chance to do a lot of different things, and the story builds up suspense effectively, to the point where you share in the anxiety and fear of her character. Those strengths make up for the implausible and occasionally unsatisfying plot turns.
Crawford's role gives her a chance to start off as a supremely confident, comfortable playwright, whose dream world is then transformed into a nightmare. She does quite a convincing job of taking her character through the joys, fears, and other turns that she experiences. It is largely thanks to her performance that the suspense build-up works especially well. By the time that the lengthy cat-and-mouse game in the last half of the movie begins, you are really thinking and feeling along with her. The crisis is built up skillfully, though again at the cost of some credibility.
This works very well the first time you see it. Watching it over again, it is easier to see through the less credible plot devices and other small flaws. But none of the flaws detract from Crawford's fine leading performance. Overall, it's a pretty good thriller and certainly well worth seeing once.