One quality backlink is far more useful than 100 directory links, there trash. They obviously help and contribute in ranking and awareness but I wouldn't focus all your time on it, get quality backlinks instead, there way more useful and impress the search engines far more.
i will suggest you to submit your site to directories and have some backlinks . link exchange is also a good source of advertising.
Yes but I would spend more time on gettin quality backlinks, doesn't always have to be exchange either, I have links to my site on Askmen.com and Starpulse.com just because they wanted to or I asked them. And Google will value a backlink on Askmen.com much higher than a shitty directory link.