Re: How much of the extra content on a disk do you watch?
If the movie was actually interestng (to me the viewer, not the movie makers...), and not just entertaining, then I *will* go through all the extras. I have noticed that people who make interesting movies also have interesting things to show and say...no big surprise. Those who don't (i.e. those who are highly paid to say others' words) mostly just compliment each other in all the space-filling extras. I'd prefer not to pay for that, never mind watch it.
Somebody mentioned Kevin Smith. I watch or listen to every single item on his discs. Pretty funny, often better than the movies even, well worth it. And somebody mentioned Kingdom of Heaven. That's a good example: anything with an historical context I watch the "documentary" extras. I'll throw in Robert Rodriguez; he explains the processes of what he does (which is pretty much some of everything) well for the layman. I like listening to Martin Scorsese talk about films. So, for me it depends who's talking sometimes. Generally anything with an historical context or technical details of the sets/production I like. I couldn't care less about CGI details and storyboards etc., a little too dry for me, though I do find the Pixar stuff like that kinda interesting (again, could be the specific people talking).
Edit: oops, got off track. I almost always watch the deleted scenes and gag reals, even for not that great films. I don't watch trailers if I have the choice, except the ones for BDs I might want to buy. I kinda forgot this is the BD section...much of the extra stuff I mentioned doesn't even exist on BD yet...