The merits of the movie itself are debated enough in other posts. I will limit myself to pointing out that as attractive as the October 2008 reissue is, there is one singular flaw, namely in the audio tracks.
True, the new extra features and the marginally better quality image as compared to the previous anamorphic version in the Hitchcock Masterpiece box set are probably enough for a die hard fan of this masterpiece to shell out for still another version of the DVD. Even the Friedkin commentary is quite enjoyable and offers some useful insights, contrary to his somewhat spotty reputation as to his commentary talents.
However, the two audio tracks are replicas of the attempt at modernizing it in the 90s by making a stereo version. The absence of the original sound effects track led to some tinkering and yielded certain strange results, most notably the double gunshots during the initial rooftop chase and a generally less-detailed aural picture. Strange that the edition in the boxset did include the original mono mix, but Universal dropped it in 2008. For those who insist on purchasing only the most perfect edition, this is not quite it then. It was not enough to stop me, but I do notice that the sound experience is slightly less interesting with this edition.
As for the improvement in image quality, it may not be visible on all systems. I did the comparison on two other systems and in one case the improvement was also noticeable, the image being sharper and the colors more vivid, while on the other one my friend and I saw no difference.
In conclusion, perhaps a Blu-Ray edition will one day bring it all together, correcting the audio deficiencies and adding HD quality, should it appear one day.