This movie was adapted and directed by William Peter Blatty, and as much love as I have for Friedkin, this is as good, and in many ways more interesting.
I highly recommend looking into this movie if you haven't seen it and Exorcist II: The Shittening turned you off the series.
This is a great movie, but I can see why so many people don't like it. Namely, it was completely fucked up by the studio. I would love to see a true director's cut.