I assume that many people who are looking at this product are doing so because they already know the film and wonder if the upgrade to Blu-ray is worth the money. Long story made short: if you have an appreciation for this movie, it is worth every penny to buy on Blu-ray.
For those people reading reviews to determine if they should watch this movie for the first time: This is the film adaptation of William Peter Blatty's novel "Legion" which he wrote as a sequel to "The Exorcist". The plot does not cleanly fall into the horror genre like its predecessor; rather it seems to fit better in a supernatural detective thriller, something closer to the noir genre. Kinderman, the detective who investigated Burke Denning's death in the first story, is back investigating a string of homicides. The only connections are Regan's exorcism and a long-dead serial killer. The book was meant to be an exploration of good and evil, of (Catholic) God's plan for us. The movie, rewritten by Blatty himself (who is notorious for revising his own work), is more narrow and simply acts as a basic search for God. This movie is far more philosophical on the surface than "The Exorcist" and therefore is slower in pace and more contemplative. Personally, I have always loved it, but it isn't for everyone.
On the Scream Factory Blu-Ray release: I could not have been happier with this release. As a fan of the film, I have always hoped that something of the cut footage would be found and restored in the form of a Director's Cut. The film is well-known for being overwhelmed by unsatisfied studio execs who just wanted another "Exorcist". While the original negatives couldn't be located, video tape of the dailies were found with a large portion of the original cut of the film still intact. Scream Factory did its best to splice those scenes back into the film (they are VHS-quality and in full-frame) and restore the film as close to Blatty's approved script as possible. The result is a 2-disc extravaganza for fans of the film. Disc 1 contains a beautiful 2k version of the theatrical cut of the film (previously the only cut available at all except for the ambitious Spicediver fan edit, which was an early, unauthorized attempt to create a director's cut) with a wonderful mastering of the sound. Included on this disc are a collection of interviews with cast and crew which, while probably available somewhere, were never on any DVD release of this film, so will probably be new to most fans. Also of note on this disc is a collection of cut footage (some of which isn't even in the Director's Cut) and outtakes which are fascinating to watch. Some of the bloopers are quite funny.
The big draw for this release of the film is Disc 2: The Director's Cut. I don't want to spoil it for anyone, but the most notable changes are the completely restored original performance by Brad Dourif as the Gemini Killer/Damian and a completely different ending with no exorcism scene. Dourif's performance in this version is far more subtle than in the theatrical cut (which was his second version of the part) and, in my opinion, more creepy. The tension stems from his almost flippant discussion of murder and evil, his casual nature as opposed to his insistent and violent theatrical version. I don't know that I prefer one approach over the other (Dourif alone or Dourif/Miller combo), as they both have their merits. The ending to this cut of the film is perhaps more bleak and open-ended. It was abrupt and jarring, and still not like the ending to the novel. Overall, I find this cut to be closer in style and nature to the novel, so it wins me over there. Whether I would say one cut of the film is superior to the other, I cannot be sure. I've spent so much time appreciating the theatrical version that I could never turn my back on it completely.
Special features on Disc 2 are plentiful, but the multi-part "Making of" feature is absolutely enlightening for fans. Everyone has their own opinion of how this movie went, with blame being predictably placed on everyone (studio people blame Blatty, Dourif blames studio...). The fact that everyone is probably correct demonstrates what a confused mess the production of this film probably was. It also paints the film's creation in a whole new light, one that breaks down the creative process of adapting a novel to screen, how actors approach their roles, how cast and crew get along, and how the industry has so many factions with various (sometimes competing) goals. Couple this with the interview with Blatty himself, which acts as a proxy for a director's commentary, and you get a fairly complete picture of the making of this film, one that is far more honest than almost any behind-the-scenes features have ever been (save for Bill Condon's self-flagellating commentary for "Candyman 2").
Overall, I would highly recommend this to anyone who is a fan of Blatty's work overall, and definitely fans of this film specifically. The Director's Cut is the long-lost version of the film we always wondered about and it was definitely worth Scream Factory's time to put it together, and for us to watch it. Once again, Scream Factory has given fans a fantastic release of a fantastic film.