Adam O'Fallon Price writes lovingly about the em dash, which I also love. (Although I like to put spaces on either side of them, which appears to be entirely wrong.) I'm sure I overuse them -- but why shouldn't I? They're awesome. I'm going to write a poem about em dashes -- stay tuned.
It might be useful to include an official definition of the em. From The Punctuation Guide: "The em dash is perhaps the most versatile punctuation mark. Depending on the context, the em dash can take the place of commas, parentheses, or colons--in each case to slightly different effect." The "slightly different" part is, to me, the em dash's appeal summarized. It is the doppelgänger of the punctuation world, a talented mimic impersonating other punctuation, but not exactly, leaving space to shade meaning. This space allows different authors to use the em dash in different ways, and so the em dash can be especially revealing of an author's style, even their character.
