Just read the book...
After the great Mr Blatty died I decided to read the book which I have never read and I bought the bluray with tons of extras and a great transfer. It also has a directors cut which i have not watched yet but it has its own page on IMDB under the books name Legion.
Better than the Exorcist? God no but it is worth a look. I seen it many years ago and just remember it was confusing. After just reading the book it all made sense. The ending was not like that in the book. I guess they figured it was called the Exorcist there had to be one. Another part of the book that was deleted was another doctor who had a big part from neurology.
The movie had several very creepy moments with the great George C. Scott hamming it up. After the horrible Exorcist 2 there was no way but up.
Worth a look.
The Exorcist- 10 out of 10.
The Exorcist III- 7 out of 10.