Not very. In general, it's extremely unlikely that it'll be the make or break point for interviews or ranking. From what I've heard from others on admission committees you can generally only hurt yourself. It does give you a chance to explain a deficit in your app or highlight interesting facts/accomplishments that can be discussion fodder on interviews.
In general, I'd say, don't say anything too controversial/political and don't be too arrogant. Other things I've been told about specific cases. Don't use quotations (and if you do, make sure you get the quote and the attribution correct). You never know when someone will be annoyed with the original quote or the way in which you interpret it. Be careful and make sure spelling and grammar are perfect (some people are really picky about that kind of thing). Don't bring up anything you wouldn't be comfortable discussing in more detail on an interview.
