I very, very rarely do that. I think the only commentary I've ever listened to was the one to the Farscape pilot.
Same with most additional content actually. Even the making of are getting more and more boring, because they basically rehash the same things over and over: we filmed that, removed this and that on the computer, enhanced this or that with the computer, added computer effects.. computer here, computer there, etc. I much prefer the making of for older movies, where they show how they did the effects practically, without little to no computer assistance.
I am even more amazed by making ofs, when they show a special effect I thought was CGI which turned out to be a mechanical or optical effect
In the beginning I listened to each audio commentary after I had watched the movie. But then my unwatched pile got bigger and my time less and I started to tag the movies, where I still needed to listen to the audio commentary. Now I have 200 profiles with still unwatched commentaries. I like the commentaries best, where a group of actors get together and tell anectodes about filming and their experiance in and after the movie. Director's commentaries are also good, especially if he/she does it together with the lead actor (then it usually doesn't get to technical). The least interesting commentaries are usually technical ones like from special effect guys. That information belongs to a decent featurette.