I had an interesting experience in college. In a film class we had to watch The Exorcist. This wasn't my first time seeing the movie (more on that in a bit). I was on the edge of my seat with every frame until a couple of guys started laughing at the special effects in the vomiting scene. Afterwards the class discussed the film and I was shocked as most of the class didn't enjoy the movie. In fact most didn't find it all the shocking or appalling. This was back in the mid 90's and sure the gore factor had been increased in the twenty plus years since The Exorcist was released, but the film still packs a powerful punch.
Perhaps the movie creeps me out so much because I first watched it when I was around twelve. Two other friends and myself stayed up late one night. After his parents went to sleep we popped in the VHS tape and hit play. Well, the movie scared the shit out of us. We stayed awake until the sun rose, each afraid to move. Maybe the flick effected us so much because we were around the same age as poor Regan. Maybe it was because we too young to be watching a film as that magnitude.
In any case, this has remained one of my all time favorite horror films. I never grow tire of it.
On a side note, about six years ago I listened the book by William Blatty while driving cross country for work and it's every bit as powerful. Read it or listen to it if you get a chance.