i guess, you should pay for the directories which are managed and edited by humans, as they pay personal attention to each listing.
I have a webdirectory where i pay attention towards each listings's traffic and requirements.
I will also do the directory submission work just for PR and banklinks. I will rather do other promotional works like submitting to various social networks like stumbleupon and myspace to bring in some free traffic.
IMO, for PR and for traffic as well. having your backlinks at high PR directories will help in building your PR. Traffic from directories tend to be targeted and these type of traffic has the highest percentage of conversion from my experience.
Directories can help with anchor text and to rank certain key phrases and also provide backlinks. Directory submission is more beneficial for new sites that have no backlinks yet and pages that are trying to rank well for long tail keywords