I don't know, I really appreciated Peter Sykes' direction. The way he used the camera gave this film a much better sense of atmosphere and pacing than most of Hammer's output. Sure the climax is a letdown, but I really love the look and feel of this one.
The last Hammer horror film is unfortunately not one of their best, an uneven adaptation of a pretty mediocre Dennis Wheatley novel. Christopher Lee and Denholm Elliott give it their all, as usual, though Richard Widmark is a bit past his prime, and Nastassja Kinski is nothing special in her film debut. It has its moments, but it's slow-paced and fairly forgettable.
Points for the bold gross-outs, but everyone except Christopher Lee seems bored (the blasé reaction to a woman being stabbed to death...case in point). The red demon muppet...which looks like a member of the Lollipop Guild from The Wizard of Oz, covered in cranberry sauce...easily steals the show.