Of the new Blu-Ray commentaries, only the Ron Moore/Michael Taylor track for Star Trek III is really worth listening to. Both have a lot of insight into Trek and what makes it work, despite neither having worked on the film. I honestly don't remember any of the other tracks, except David Carson gets a few things wrong on the Generations track and he and Manny Coto (who acts as moderator) go off on a few too many tangents.
On the other hand... of the original commentaries, you can't go wrong with Nick Meyer's track for Star Trek II and the Moore/Braga tracks for Generations and First Contact.
Meyer is as erudite as ever (I wish I had a nickel for all the references and names he drops in his interview for the expanded soundtrack album!) and he has a lot of good insight into how to tell a story, work with actors (the more takes Shatner did, the better he got), etc.
Generations is one of the most candid commentaries I've ever heard. Neither of them are completely happy with the way the film came out. They feel the Nexus wasn't properly defined or explored, the pairing of Kirk and Picard wasn't nearly as big as it should've been, and neither has an answer for any of the time travel plot holes or things like Picard's "echo." ("If Picard has an echo in the Nexus, does Kirk have one, too?") What's even more interesting are their anecdotes about working on the show. Braga is ashamed of their reliance on technobabble and both admit that no one really knew what Guinan's powers were or even the extent of Troi's powers for that matter. Moore says they basically resorted to making her a glorified cruise ship social director.