"The Elements of Style by Strunk and White discusses the em dash, defining it as a mark stronger than a comma, less formal than a colon, and more relaxed than parentheses, used to set off an abrupt break or interruption or to announce a long appositive or summary. However, the authors advise using it sparingly, only when a more common mark of punctuation seems inadequate."

Right on

Wow, this board has everything—never thought I'd see a reference to Strunk & White, as it rests on a shelf 12 inches from my head all day