Do you overuse em dashes? I do too. It’s because they are awesomely useful:
A dash can replace a comma to add emphasis—“She was born in Philadelphia—and a beautiful day it was.”
A dash can replace a colon, to introduce the information that follows it—“Her parents always remember her birthday—July 4, 1976.”
A dash can also replace parentheses to indicate, well, parenthetical material—“On the day she was born—it was also the 200th birthday of the United States—her father wore red, white, and blue.”
A dash can be used to indicate the interruption of one thought with another—and its resumption —“When she was twelve she asked—because she was curious—why there were always fireworks on her birthday.”
A dash can indicate an abrupt change—“She was supposed to be a boy—but she wasn’t.”
Merrill Perlman: On, Dasher! | Columbia Journalism Review
Also, here’s how to make an em dash on a Mac:
Option + Shift + – (hyphen)
And here’s a case against the em dash from—who else?—the contrarians at Slate.