Where would writers be without the helpful em-dash?†
A dash, the style manuals helpfully state, indicates a break in the sentence, or (as part of a pair) encloses a parenthetical statement. In practice, this makes the dash an informal replacement for several other pieces of punctuation: parentheses, the colon, the semicolon, even ellipses.
It’s the protean nature of the dash that presents the greatest hazard. Being so useful in a variety of different situations, it’s vulnerable to overuse. A writer who isn’t careful can end up with a page full of dash-filled sentences, which lends a sort of panting urgency to the prose that usually isn’t wanted. The best general advice I ever read on the subject was a stricture I encountered in a media fanzine back in the pre-internet era, and it ran something like this: “If you use more than two dashes in one paragraph, you aren’t allowed to use any at all in the next. So there.”‡
There’s a higher-level version of the same problem, involving semicolons. There are some writers — I plead guilty here — who like semicolons entirely too much. If I’m not careful, I can find myself committing a three-sentence paragraph where all three sentences feature independent clauses joined together by semicolons. At that point, I have to force myself to take an axe to at least two of those sentences and break them back up into their component clauses.
Sometimes, though, I cheat, and replace one of the semicolons with a dash instead.
†There’s also a shorter version, the en-dash, but its uses are much more restricted and frankly, if you use a hyphen nobody’s going to call you on it. Well, maybe the typesetter, but unless you’re being your own desktop publisher, you aren’t likely to ever meet him or her.
‡I believe the zine reviewer in question was noted Star Trek fan Paula Block, but at this remove I can’t be sure. Whoever it was, I owe her for the words of wisdom.
Dr. Doyle's Blog 
I can restrain myself with the dashes, but I’m just as devoted to the semicolon as you are, and I have to be just as ruthless when I edit. And in fiction, I’m not too bad with the parentheses, but in a blog post? I have to force myself to go back and weed them out.
I’ve been told I speak in parentheses. I don’t doubt it.
Yeah, I’m an m-dash addict. I have stories from my high school creative writing class that have notes like “you can use dashes for parens and semicolons, but not both in the same sentence.”
I’ve gotten a little better since then, but I do still love my dashes.
It might be a rule unique to British English, however I was taught the em-dash is not interchangeable with other punctuation. It is used instead of them if the clause is a negation or restriction of the main sentence whereas they are used where it is a clarification or expansion. For example:
It is a well known fact – except among those who have taken the time to research the subject – that….
And not:
It is a well known fact (except among those who have taken the time to research the subject) that….
I certainly use both many semi-colons and many dashes; it is possible this formality with dashes keeps my usage of each within acceptable limits – except when dealing with Microsoft who seem to believe semi-colons are incorrect in all circumstances.
Punctuation is so variable — by language, by region, even by publisher — that a lot of time the only good advice seems to be “pick a style and be consistent” and “don’t overdo the special effects.”
(And let the copyeditor worry about the edge cases.)
I have to watch myself; I love em-dashes entirely too much. I often need to rewrite things when I realize every third sentence has an em-dash.
They do say that the first step is knowing that you have a problem. (“Hello. I’m Debra, and I’m a semicolon addict.”)