I enjoy commentaries for a few reasons... first, as a writer, because I like referencing some of the subtler points in my stories, to see if anyone else picked up on them (plus I may forget myself in time!). Second, as a reader, because I like reading about how others put things together, so I figure maybe others would like to read about me. It might even help someone else out, or make them feel better about their own struggles. And finally, because VERY few people I know read my fiction, but some will read my non-fiction, meaning my commentary posts tend to be the most popular. (Yeah, mixed feelings there.)
In terms of where it belongs, I'm inclined to say it's own post. I mean, if it's a couple sentences, it could be a comment... but you'd have to read the post to get that far. So the proximity is only really a positive if people are already engaged in the material - granted, I suppose you can check your stats or other comments to see if that's the case. I'm against the "italics at the start" idea, it pulls me out of the story. (I mind it a bit less at the end, but at that point it could simply be a comment!) And the final benefit of it's own post is that people who don't read the story can still check it out (see the end of my last paragraph). I used to do a commentary post for T&T every other week; I've since cut that back.