It depends of quality of the directory. I have a lot accepted submittions but Google only indexed a few. By the way, all backlinks are dofollow.
Directory submission has a weak effect on SEO at best. Still, I couldn't do without it. I use it in the early stages of SEO. As 5starpix said, niche directories are better but general ones are OK, too. Then, I focus more on quality 2-way and 3-way link trades. It's just a matter of e-mailing webmasters and using the forum marketplace. I also like finding review sites who will give an honest analysis of my site.
Directory submission is now become old SEO practice. Search engines already flaged on this type of SEO. Only few directories are useful like that. It it surprise that still people & SEO companies are submitting their sites to 100's even 1000's directories, that's only spam.
Directory submissions can help your site to achieve high ranking to some extent. Google will hardly show you 2/3 directories as your backlinks after you submit in so many places. For generating traffic I think we should not compare it with submission in social bookmarking or networking sites. They're much more helpful for getting target traffic to your site.