Directory submission will not help you to increase good traffic to any web site but it helps you to increase good Quality back link.
I write articles to post on article directory not for traffic from that but to make more backlinks. search engine will give my website in a good page rank if i have many backlinks from that directories.
I get limited traffic from directories but I would still add to a directory when I come across one while browsing
I've found the traffic from web directories insignificant,however I've invested only on general ones till date. I 'll soon be submitting to niche directories and I think the outcome will be better.