I like the idea of ripping commentary tracks to MP3. That's a great idea! I've actually ripped the MOVIE dialog to MP3, but never the commentary. Consider the idea now stolen by me

Kevin Smith: With Smith and Affleck on a commentary track you can't go wrong. Jay Mewes is a useless junkie on the tracks. Perhaps the best KS commentary track though for sheer amusement was "Clerks: The Animated Series". I really enjoyed those. Chasing Amy is okay. Mallrats is probably the best, followed by the big Dogma track. (Shame Jay and Silent Bob Strike Back didn't go the two commentary track route.)
Bad tracks? Well "The Replacement Killers" put me off right away when the director starts the commentary track by quite obviously asking the person recording "What should I talk about then?" I turned it off right there and then

The Costner/Robbins commentary track on Bull Durham is bugging me (about half way through) as they seem to spend a lot of time just watching the movie (even after Costner says "we should be careful that we don't just start watching the movie").
Pearl Harbor 4 disker: You have Affleck and Hartnett who are hysterical, but then it goes rambling on with Michael Bay (who if I recall is actually on TWO tracks) and Alec Baldwin. Now I love Alec Baldwin but DAMN the man is dull on this track.
I don't recall ever hearing "But I think we pulled it off", but I gotta agree with Stiller. A lot of them are "Oh I love this scene... Oh I love this scene too..." Tiresome. (Incidentally, Zoolander's commentary track from Stiller is great.)
My biggest peeve is LONG PERIODS OF SILENCE!