For those of us raised in the world of proper English grammar and punctuation, where formal writing eschewed things like hastily struck lines to break up phrases, the dash thing can be confusing. However, I've never been a big fan of the semi-colon, and I now embrace dashes of all stripes--- em, en, and the plain old hyphen.
But don't trust me on this. Cheryl Klein, world famous editor extraordinaire, tells it all on this blogpost.
Related post: Cheryl Klein on commas