I think it's better done with spacing, as in "to-day - a good day - is the middle of the week", when you're not being anal enough to use &emdash; or Unicode literals etc.
I think the non-spaced -- looks a lot like double-spacing after full stops (periods): quite old-fashioned. Reminds me of books from the 50s I read in the 80s.
Many of these typographical conventions are fashion and culture based, anyhow. Quotation marks are especially diverse, with the << >> in French, the em-dashes used by Joyce, etc.
