You can check incoming and outgoing traffic stats under listings to decide how much traffic You can get from my directory - www.portachi.net (data for about 2 months
If u submit into few thousands directory u'll get very nice traffic (but this is not a BIG traffic weapon just increase ur backlinks number).
IMHO much more traffic and PR-juice You will get from TOP lists (for example based on this script - Aardvark Topsites PHP)
Many PHPld based directory have traffic at SUBMIT page, so they will never give you traffic. But few directories like my WebZone directory will give you traffic, Just tro to submit the link over there. most of the directories are made for Links and not for traffic, one of my site is listed at ODP, but i never get huge traffic from there.
Submitting to right category in directories can also increase traffic, becuase those who looking for shopping site wont go look in Health category. What I mean is I have noticed in my directory while reviewing links that most of the site owners don't care submitting site in right category and most of the directory owners don't care reviewing so result is no traffic. who knows shopping site is in health center. It was just for information and i guess it also matters.