I only do SEO as part of other business duties, so I can guarantee you'll find some better qualified opinions. But, before I was too much time or any resources trying to get into directories, I check their Google page rank using a site like http://www.prchecker.info/check_page_rank.php

While I'll never turn down a free listing and I do always try and vary the description, directories with high Google page ranks are a priority for me in trying to move towards the front page of search results.