Not scary at all
I just read a peace of the Exorcist being the most scary movie here:
http://www.tasteofcinema.com/2017/8-reasons-why-the-exorcist-is-the-scariest-movie-ever-made/
I went to go see this movie for the 30 year anniversary back in 2003 not having seen it before only knowing it was a classic that made a lot of impact in the horror genre.
All I remember was me and my friend not being able to hold it in, we were laughing our ass off for the entire thing. To be fair, of all the people sitting in there we were probably the only ones laughing. It was chocking how bad it was and I guess time had passed on this one. True comedy peace, we left the theater with nothing but bad things to say about it expecting to see something else.