What's odd about _Exorcism_ is that it's in no way similar to _The Exorcist_... at all. I'm joking, it's a shameless rip off, with some _Rosemary's Baby_ thrown in for good measure.
Even though this is pure plagiarism, it didn't have to be horrible, yet it is, which is its worse offense.
The acting is over the top, the story is convoluted, the movie wastes too much time with people just standing around talking. We don't even see Leila in full possession mode until the third act. The tragedy is, the movie does start to become interesting when we finally get to the moment of exorcism; trouble is, it ends right after that.
The only saving graces here are: the great Paul Naschy, a good amount of nudity, and Mercedes Molina - once she is in full demonic makeup, she looks great; like she belongs in a Fulci film.
The only reason I watched this is because of Naschy, and for that, I don't regret it. Maybe if I had never seen _The Exorcist_, this may have been a 6/10; trouble is, that movie's a classic, one of the best horror movies ever made, and _Exorcism_ is just a pitiful copy.