I've been a fan of the original Exorcist for over 10 years now, it probably would have been longer but as you may or may not know the film was banned in the UK for many years & unless you were "in the know" it was nigh on impossible to get a copy on video.
Anyway, here's my 2 cents on the new "version you've never seen before" versus "the original version"
On the whole I was very happy with the changes that had been made, I think Friedkin has made some very shrewd additions & tweaks in the newer version. There are some nice touches especially the images of Pazuzu popping up when you don't expect it to, leading to the "did I just see that?" scenario. Also there is a new segment of Regan's medical tests, my only gripe with this was that it was put in the movie too early, no mention had really been made that she was ill, next thing you know she is swearing at doctors & acting like a moody teen. I think these scenes would have been better placed just after the party scene or there abouts.
The spider walk scene was a bit cheesy in my opnion, I had seen the original outake & it was much better, I just didn't get why she had to have a mouthful of blood when she reaches the bottom of the stairs? The Exorcist has never been a about gore & cheap shocks, and I felt it was just a tacky shot that wasn't needed.
I really liked the new ending though, nice exchange with Kinderman & Dyer, not as much as a downer as the original version.
Anyway, on the whole I think this newer version is worth a look, especially if you are a fan of the original, I guess if you haven't seen either it doesn't matter which one you watch. ut in a time where versions of older films are almost changing beyond recognition (Star Wars Trilogy anyone?) It's good to see that subtle tweaks can still be as effective.