Lots of times the decision about whether to buy the movie or not will be decided by the extras on it. I love commentaries especially if some of the cast is in it. I agree with you. I've seen LOTR soooo many times because I've watched all the commentaries. Love them.
I watch at least some of the special features. If I enjoyed the movie, I'd like to know more. Usually for documentaries and "based on real events" films, but sometimes for regular films. I find it really adds depth and texture to my understanding the film. Especially the "real life people" commentary in movies like "Black Hawk Down", "Why We Fight", "Apollo 13" added a LOT of impact and comprehensive understanding. It was like having the movie slow-cooked to a very rich consistency. I find after I wait a few days and watch the movie again (commentary off), the movie has a great deal of richness and complexity like it's grandma's sloooooww cooked family secret...amazing!
Usually I'll watch to trailer or previews before the movie (mostly it just passes some time while I make dinner or popcorn).
The actors commentary on the Lord of the Rings: Fellowship of the Ring (special edition) is funny! The actors have a good humor about different parts of the film. Similar for "Good Will Hunting".
The gag reels are always fun! Sometimes I'll go through deleted scenes or "the making of".
p.s. - I'm really looking forward to a "gag reel" for Transformers!