They will help with backlinks, but should only be a small part of your marketing campaign. I think article submission, social bookmarking, and guest posting are more important, but submitting to a few directories won't hurt. The only directory I would pay for is yahoo, other than that if it is not free more than likely it is not worth your time. Just my thoughts on directories though. One more thing if the directory is in your niche then it is a good thing to submit your site and perhaps pay if the
PR is high enough.