“Use spaced en dashes – rather than close-set em dashes or spaced hyphens – to set off phrases.”
Robert Bringhurst, The Elements of Typographic Style, p.80 (via DI). Bringhurst continues:
The em dash is the nineteenth-century standard, still prescribed in many editorial style books, but the em dash is too long for use with the best text faces. Like the oversized space between sentences, it belongs to the padded and corseted aesthetic of Victorian typography.
