Quite often.
It depends on who's doing the said commentary. Some directors are really interesting, explaining all sorts of things while telling very funny anecdotes about the actors or locations while others are dreadful bores.
Sometimes it's the actors with or without the director. View Askew's commentaries are a legendary type of these. You can hear Kevin Smith, Jason Mewes, Ben Affleck, Ethan Ambry, Jason Lee and others, sometimes under the influence, comment, b*tch and joke about such classics as Clerks, Mallrats, Chasing Amy or Dogma. Usually, actors in comedy do pretty good commentaries. They're worth a watch. But sometimes you expect a lot and it turns out to be horrible. Owen Wilson and Vince Vaughn do the commentary thing for Wedding Crashers. I tought to myself, ouh! this should be great! It sucks. They were tedious. Somehow, funny on screen didn't translate to the extra audio track at all.