If I like the movie, I usually listen to the commentary. Sometimes they're interesting, sometimes they're not. I've enjoyed most of the ones I've sat through, though.
This is pretty much my commentary experience in a nutshell. Watch as many as I can ('cause you know, when you rent a DVD, there's only so much time... which is why I end up buying so d*mn many) and enjoy. Aronofsky's PI and Requiem commentaries are brilliant, and Kevin Smith and Martin Scorcese are always a treat.
Two really strange ones I've heard:
The Cannibal! The Musical commentary--Trey Parker, Matt Stone, and all the usual suspects getting drunk while talking about the movie. Not even an hour into the flick they start talking about going to the tittie bar.

And even with how funny it is, you still get some great tidbits about the struggles of making movies with no money, no professionals, and no time.
The Sleepaway Camp commentary--remember this movie? When they released it on DVD and it actually had a commentary, I freaked out. I hadn't seen it in about ten years, so my husband and I got together a couple of friends who'd never seen it and watched the movie, which was universally proclaimed to be fairly lame--you know how it is, movies lose their luster when they're dated and crappy--and then we got the urge to listen to the commentary, mostly to figure out where that last shocking image (you know the one I'm talking about) came from. The commentary itself was so strange, we wound up watching the whole thing. There's nothing like a nobody director, his nobody actress, and the creator of the official SC fan site (seriously) talking seriously about character motivation and actors you'll never see again who appear in the movie itself for about three minutes ("Yeah, he was such a professional. A great actor.") Unbelievable.
'Kay. Babbling now.
Mary Lo