This is one of the great all-time fell-on-its butt movies. I especially like when the doc is having a heart attack and rather than call an ambulance or anything, they have Linda Blair hooked up to the electroshock skullcap and have her overpower the demon with sheer sincerity.
I thought we were in for more of the same when EIII opens to George C. Scott angrily lecturing his detectives about Shakespeare, but they soon got their oars in the water and I can easily see why this was Jeff Dahmer's all-time favorite movie.