Both of the Evil Dead 1 commentaries are excellent, but the Bruce Campbell one was especially good. Very entertaining to hear the story-behind-the-story.
The Re-animator commentary was good, something about creating low-budget ultraviolent horror movies makes for good stories 🙂
The Simpsons disc 1 commentaries are interesting and worth a listen.
I gave up on the Conan commentary are about 10 minutes, Ahnold and John Milius seemed to be grasping for things to say -- maybe if I'd fast-forwarded at bit it would get better.
So I'd say they can be good, indifferent, or boring -- depends on the movie, the people, and what they have to say about it. I was quite disappointed that Tank Girl and Buckaroo Banzai didn't have them 🙂