How often do you use the em dash in your writing?
If you use it, you most likely overuse it. And at my food blog Casual Kitchen, I'm as guilty as anyone. What's worse, I don't even use the official punctuation--I never did figure out the CTRL+SHIFT+something combination to make an official em dash--so I substitute two hyphens instead.
Think of the dash as a comma--on steroids. It indicates a longer pause than a comma--a more substantial break between ideas. You can use it to set off a clause or a parenthetical thought--but a with more emphatic break than you'd get by using mere commas.
What's the problem--the real problem--with the em dash? It's this--when you emphasize everything, you emphasize nothing. And therefore your impact--if your writing had any to begin with--is ruined.