There are 4 episodes that have commentaries on them:
"A Tale of Two Cities" (Damon Lindelof & Elizabeth Mitchell)
"I Do" (Carlton Cuse, Evangeline Lilly, & Josh Holloway)
"Expose" (Edward Kitsis & Adam Horowitz)
"The Man Behind the Curtain" (Damon Lindelof, Carlton Cuse, & Michael Emerson)
I have watched 3 (all but "Expose"). "Tale" is a lot of Damon being goofy and Elizabeth Mitchell (Juliet) laughing at him. Not really much insight. Lots of discussion about the 6 episode block, the Hydra set, etc. No good giveaways. "I Do" is mostly a discussion about Kate and her relationships with Sawyer & Jack, Evangeline's & Josh's processes as actors, and working on the Hydra set. Again, this is more of a "behind the scenes of filming" and character study than a plot revelation.
"The Man Behind the Curtain", otoh, is a MUST see for people who want more to sink their teeth into regarding the mythology of the island. There were LOTS of good nuggets here. I'll throw out what I remember...
* Annie's story is not done. She was HUGE in Ben's life and we haven't seen the end of her.
* We didn't really see Ben's motive for participating in the purge, but we will.
* Horace and Olivia will return (in flashback, of course) and are also important to the Dharma story.
* The volcano that Olivia was replicating in class represents a real one on the island that is of "seismic" (pun by Damon intended) importance.
* Damon described the relationship between Alpert and Ben as like the Panchen Lama and the Dalai Lama. Apparently, after the Dalai Lama dies, it is the Panchen Lama's job to meditate and find where the Dalai Lama's spirit has been reincarnated (and vice versa, thus creating sort of a chicken and egg scenario on the island). Alpert would be the Panchen Lama to Ben's Dalai Lama.
* The ash circle around Jacob's cabin serves a purpose. They talked about Ben having certain powers over Jacob and Jacob having certain powers over Ben.
* Damon said that Ben's penchant for using rabbits is likely tied to fertility experiments, but Carlton added that they are probably numbered to be able to tell them apart (possibly referring to the Orchid video?).
* The journal that Ben is writing in at the beginning of the episode was actually written by Michael Emerson and was initially just supposed to be an unimportant prop. Damon & Carlton said that when they realized that the journal would be visible (and able to be screen captured by DVRs) they had to read it to make sure there wasn't any "bad" information there. They said they were amazed at how spot-on it was. This seemed to surprise Michael Emerson, too.
* Damon and Carlton also added that they are continually amazed watching dailies of Michael Emerson's acting choices with Ben because he plays them differently, but oftentimes BETTER, than was the original intent of the script. Emerson said that they'll do multiple takes of the same scene and he's been given the liberty of trying different things with each take. They used the example of the "You do remember birthdays" line he delivers to Alpert as one that could be done many ways, and that Emerson doesn't really know yet what he meant by that line.
There's more I've forgotten, but I WILL be watching this commentary again. I don't think I've ever said that about a DVD. Usually I'm one-and-done with them if I even watch them at all. This is one where you'll want a notebook handy.