[shakes head] DVD extras are a huge factor in my buying a film again, in fact I rarely check to see if the picture and sound have been improved, probably because I take it for granted that it probably has better PQ and sound, or even if the movies exactly the same as before I'm only really buying it again for the extras. I've double-dipped 110 times in 7 years and nearly all because of the addition of extras.
It bemuses me when people say they don't give a crap about background material or any info at all on a film, as a film fan I can't imagine anything more fascinating. The inclusion of extras, particularly commentaries was the single biggest factor that finally made me switch to DVD in 1999. I feel it really enhances my enjoyment of watching a movie knowing something about the history of how it came about.
That said I've ordered Tora! Tora! Tora!, a double-dip, my current copy only has the commentary, looking forward to seeing the video material on the subject and I'll have to order Patton too, oh and The Dirty Dozen now has a slew of extras too, keep em coming.