Overall, this is an effectively creepy story that is more contained than his other stories of horror breaking out everywhere. The explanations behind it all are interesting enough. The gore is grotesque enough. Still, it’s only slightly better than an average film.
If you want to see the state of low to mid-budget horror in the late 70’s, this is as good a place as any to start.
It’s effective, for sure. Not my gig, even slightly, but it is not a bad film. It might even be considered good for the time.