According to Wikipedia, "The em dash (—), or m dash, m-rule, etc., often demarcates a parenthetical thought or some similar interpolation."
I recently submitted a draft of a nonfiction piece to my editor. And much to my chagrin, one of her comments was about my beloved em dash. I was chagrined mostly because I had removed a half-dozen of the dashes before submitting because a friend of mine noted that I had used so many.
When I confessed this to my editor, she replied how much SHE likes the em dash in her own writing--how's that for karma!
It's important to know your own weaknesses! I'm also guilty of overusing commas, but that's another post...
Any one else willing to stand up and admit your em dash love? (See, I only used it once in this post!)