I was visiting the Catholic college I graduated from shortly after the movie came out. I was talking with one of the priests I knew and the topic of the movie came up. He told me that he knew the priest who did exorcism and that it was a real case of possession. This is not something I would have expected one of my former professors to say unless he really believed it. I later mentioned this to a colleague at work and he said he knew the younger priest involved in the case. Both said that while the movie had many sensational things happening, there was no rational explanation for what actually occurred during those weeks of exorcism.
Thanks for your post. From what I have read, the movie was sensationalized, but not much.