(In the Lord of the Rings commentary, Billy Boyd and Dominic Monaghan (Pippin and Merry) are worth listening to, being amusing people in their own right so to speak. My favorite bit is when, Hugo Weaving on-screen, they whisper, "It's . . . it's . . . the guy from the Matrix!")
Scripted commentary is when a professor of film or an expert in something reads a lecture. These can be fascinating, and they are far, far preferable to the written biographical texts sometimes included on DVDs (which I can never read). The commentary on Brigham Young (1940), for instance, is especially good. The problem here, of course, is that it's a lecture. The Brigham Young commentary made a real effort to have the lecture correspond to what you see on the screen, but the last commentary I listened to (for that great black & white film, Laura), while interesting, didn't correspond at all to the images. Which was kind of pointless. The reason I watch movies with commentary is because I want to hear the inside scoop on how a scene was put together: how many takes were involved; whether there were prior versions. (The one thing I learned from the Laura commentary was that there was a director before Preminger who had shot some footage, but Preminger had a different concept of how the film should look and eventually, scholars think, reshot almost all of the first director's material. The main thing that struck me from the commentary was how backstabbing and nasty Hollywood was--makes the Mafia look frivolous and kind-hearted in comparison. It's probably still the same, only there's more press so directors have to try harder to pretend to be nice.)
The thing I detest the most with scripted and unscripted commentary is the commentator who says, "There's Dana Andrews. Now he's walking across the floor." Yes, I can see that. I'm watching the movie! Dean Cain does this in his commentary for an episode he wrote for Lois & Clark. However, there's something incredibly guileless about Dean Cain (despite the fact that he played Scott Peterson in the made-for-TV movie). "There's me," he says at one point, and you've got to forgive so much well-meaning astonishment. I turned it off because he kept doing it: "There's Lane Smith." "There's Justin Whalin." But I put that down to the don't-expect-actors-to-be-intellectuals-and-amazing-conversationalists issue referred to above.
On the other hand, the Laura DVD has two commentaries, and the other is awful. The one I've been referring to is by Rudy Behlmer. The other commentary starts almost exactly the way I've described ("There's Dana Andrews. He's walking across the floor.") It's ridiculous. At least Behlmer says, upfront, "If you haven't watched this before, go watch it first." The commentator is not supposed to be a narrator. The whole point of a movie, especially a great movie like Laura, is that it is an entity unto itself: not flawless but whole. If a director puts in voice-over, it's because the director wants that voice-over. If a director doesn't put in voice-over (a la, the second Blade Runner), it's because the director doesn't want voice-over. It is not up to the commentator to supply it!
Frankly, I've given up on commentary. There's very few commentaries that actually do what I want: talk about the craft. The commentary for Other Side of Heaven is actually the best I've heard; the commentator addresses different parts of the movie (while you are watching them): why certain decisions were made, etc. etc. I wish M. Night Shyamalan would do commentary (especially on Signs, which I want to see again, but it's way too scary to watch without commentary: really, it scared me way more than The Village or Sixth Sense), but maybe a certain kind of director feels there shouldn't be commentary at all. Which I can respect.