I had heard about this movie for years as a sort of Proto-slasher film. Coming two years before Halloween but a year after Texas Chainsaw Massacre, this one was meant to be unique because it is based on a true story.
Once I finally found an..ahem...collector's edition of this film at a comic book convention, I was able to take it in. The movie is shot with minimum competence and looks a lot like an MST3K vehicle. Sets have no decoration, just blank walls and maybe a single picture hanging somewhere. The acting is pretty stilted and the action is kind of goofy (one lady gets killed by a knife on the end of a trombone).
The idea of the killer is kind of interesting. They call him the Phantom and he wears a hood over his head. Unlike other slashers, he is just as likely to shoot you in the face as stab you. The way the investigation proceeds seems like it could have been the way a real case went (with the established pattern of the killing and the psychological insights into the killer that help not a bit). The lead investigator is called the Lone Wolf by his peers. You have the recipe for some good fiction here if only the execution weren't lacking.
As a curiosity, this movie is a fascinating example of the burgeoning slasher genre. As an actual movie you should sit down and enjoy...eh, not so much.