Noreen Malone rails against the em dash:
The problem with the dash—as you may have noticed!—is that it discourages truly efficient writing. It also—and this might be its worst sin—disrupts the flow of a sentence. Don't you find it annoying—and you can tell me if you do, I won't be hurt—when a writer inserts a thought into the midst of another one that's not yet complete?
The Dish 