There is nothing to tell about it. Those are just directories with good PR. There's nothing so special to it.
Page rank 7 directories are good, Google rate the site from 0 to 10 and also the content on the site is very important not page rank.
What exactly you want to know about PR 7 dir.? Having backlink from them good for your site SEO. Google trust those sites which have high PR becuase Google uses Page Rank to determine whether or not a website is an authoritative.