This movie is a must see, IF you've read the story and like it, and IF you've seen the other adaptation, "The Haunted Palace" with Vincent Price. Sure, this story is a bit different than the book. It's set in the modern day, and Charles Ward is a well-paid chemist at Belmar Cosmetics, not a young antiquarian débutante. And instead of Doctor Willet being the principle investigator, John Marsh P.I. is (nice nod to the Innsmouth stories with that last name).
Aside from those differences necessary to bring this into the modern day, and aside from a very slight difference in how Joseph Curwen is ultimately dealt with, this follows the story in the book. It's all there: the portrait, the neighbor Fenner, the house in Pawtucket, and of course the underground labs of J.C. Curwen. There are story sequences set in Colonial times to build the story as well, and they are nicely done. But the real crowning glory of this movie is the sets they built for Curwens underground lab. They are MARVELOUS. Everything is there: the sanity blasting carvings, the "mistakes and screw-ups" raised from Imperfect salts, and the jars of Materia.
I highly recommend this movie. I'm still treasuring my copy on Laser Disk and hoping that it someday comes out on DVD. Production is top notch, as is the music and of course the story.