It depends on the commentary, who is talking, and how it is edited. For example, the Generations and First Contact commentaries are great, because BB and RDM don't hold back and are pretty easy going about it. Same thing with X-Men: The Last Stand. You can tell those guys had a good time on the movie and are generally excited to do the commentary.
Then you have the boring ass ones where you can tell the actor/whomever doesn't like to be there, like Pierce Bronson's Die Another Day one. It seemed like he went in there totally unprepared and didn't really care. "Oh, that's me running. There I am again. Hey, random story about my kid."
I also dislike the pieced together commentaries, where various members of the crew pop-in at certain parts and then leave. I like it when everyone is there from beginning to end (also, I find it a bit of a cheat when they take audio samples from other interviews and place them in the commentaries). Star Trek: The Motion Picture is like that, as are the Star Wars movies, but for some reason, the OT commentaries
are pretty good and seem to be an exception to my dislike.