[DiscoNinja]

A totally different approach of directory submitting can be used. I figured to describe it here and people can use which ever they want:
The goal here of submitting your blogs is just increasing the SERP, nothing else. Doesn't matter at all how much traffic the directory will send. It can be additional positive side effect, but it is not a primary goal.
Second thing is forget about directory PR. While finding directories for submitting you just check the search engine ranking of those directories for a desired keywords. Ok this indirectly and partially is influenced by directory's PR, but still this is not a primary parameter for finding the directories.
Let say you are building a blog network in lesbian niche. You find all good keywords in that niche. Let say you found 10 (for example:free lesbian porn, free lesbian videos, lesbian kissing, etc.). Now go to Google and search for first keyword: free lesbian porn. Now in received results find all sites that receive blog submissions and save a list of them in a text file. Repeat this for all the keywords you are going to use and gather all the best ranked directories for those keywords. It is likely that you will not find much directories so you can try to search in Google for keywords lesbian, lesbian sex, lesbian porn, etc. In this way you can find some more relevant directories. To find all the directories can be time consuming, but it is not so horrible and you have to do it only once. After that you just create a lot of blogs in that one niche submit them to all the dirs you have found and inter link them how ever you want. They will all be very relevant, so you will not have to search for quality link trades, you can trade with your self.
So in this way you will have a list of blog directories which suites the most for your blog network because you will be submitting to dirs which Google likes the most in your niche. It is better to have a link from a PR2 directory which is showing in first 5 pages of SERP for a desired keywords that from a PR7 directory which is showing on page 90 for your keywords. And it is not very important what is Alexa ranking of the directory. You are not chasing a traffic from directories (because it is just temporary), you should be after SEO traffic. The more Google loves the directory when it comes to your keywords, if you submit there, it will love your blog also.
In my opinion this is how you can squeeze out the maximum benefit from submitting to directories.