This is a few weeks old, but check out this good, and funny, piece by Noreen Malone on the “em-dash” over at Slate:

The problem with the dash—as you may have noticed!—is that it discourages truly efficient writing. It also—and this might be its worst sin—disrupts the flow of a sentence. Don’t you find it annoying—and you can tell me if you do, I won’t be hurt—when a writer inserts a thought into the midst of another one that’s not yet complete? Strunk and White—who must always be mentioned in articles such as this one—counsel against overusing the dash as well: “Use a dash only when a more common mark of punctuation seems inadequate.” Who are we, we modern writers, to pass judgment—and with such shocking frequency—on these more simple forms of punctuation—the workmanlike comma, the stalwart colon, the taken-for-granted period? (One colleague—arguing strenuously that certain occasions call for the dash instead of other punctuation, for purposes of tone—told me he thinks of the parenthesis as a whisper, and the dash as a way of calling attention to a phrase. As for what I think of his observation—well, consider how I have chosen to offset it.)