well,... depends what kind of directory your talking about! i get some good traffic from DMoz and some others, while others are just good backlinks! M.
Very few and almost nothing traffic gained from submit to directories,. The best is viral, it will boost you everything, good for SERP too.
I have submitted my site to thousands of directories and have seen very little traffic from them over the years. I wouldn't rely on them for traffic, rather they are better for building your link network. I mean how often do you go to a directory and search for sites, you would just use a full blown search engine.
I think in regards to traffic, Google and Facebook, and, maybe, blogger are the way to go. As for directories, DMOZ seems to be the only one worth getting into. Had plenty of backlinks and some, though not remarkable, rankings increase from DMOZ.
don't expect more traffic from directories.... choose digg, reddit and other social bookmarks for good traffic...
Directories are for PR only, not for traffic. Backlinks from related websites, can drive you a traffic.
From Directories you will get good backlink, from High Page Rank directory if your site is approved you will get PR and traffic from Dmoz dir but directories help in getting rank in google.
To state an iteresting opinion traffic from directories to english-language sites I run is almost zero. But in Greece where I live, traffic fro the 3 most impotant directories to my greek sites was quite big when I started them. It really gave them a boost at first. Of course after a websites starts having serious traffic directories percentage is minimized. Has to do with the small country and small number of greek sites i guess (less compared to english - speaking websited that is)
directories will not give you web traffic but the through the directories you will get a back links and your web pages will cached by search engine and it will give you a web traffic to your site.