I am not sure its the PR of the directoy but the quality of contents that works best. The test I tend to conduct to see if a directory is worth submitting to or not is to date a page withing the directory, post it in search engine, if the page on its own is ranked that is a potentially good directory or if a sub page of the directory resolves for a particular keyword the its worth example of that is if you type the keyword "uk dating web directory" into google UK, you get the dating direcory section of wura directory and the directory itself is now high PR but quite a lot of its sub pages rank very well
Free Link submission directories are waste of time it is long period for approved links . High PR Paid Directories only better for improve backlinks
Its still work, doing it is registering your site to a known and well indexed directories can be more like commenting on your favorite site. The point of that is to make backlinks.