Link building is not all about transferring PageRank. Don’t get caught in the trap of basing your decision on high PageRank score alone. There are other considerations to be taken into account.

For example, your backlinks need to represent a range of importance scores (PageRank) so that Google doesn’t construe your link network as unnatural. Building links exclusively or mostly from high PageRank endowed sites may flag your site for artificially trying to boost your PageRank. And do you really want to attract additional scrutiny?

For long term benefit and security, sites that are selected for inbound links should be from an on-topic neighborhood, have aged domains, and if possible, have .edu and .gov sites in there. The list of sites needs to be analyzed to ensure that there are no technical limitations that slow the flow of “link gain” (e.g. PageRank). For example, the directory Gimpsy.com has let pages with session IDs (“PHPSESSID”) in the URLs slip into the indices, which makes it less ideal as a backlink.

In general, all links help (unless from “bad neighborhoods”), regardless of their PageRank. Some of the links NEED to be topically-relevant or your site is going to appear unfocused and the links won’t appear to have been “earned,” but instead bought, borrowed, bartered or stolen.

Directory submissions should be a component of your link building strategy, but don’t put too much emphasis on them. As Stuntdubl says, you need to balance the link equation and not rely too heavily on directories, and you need to spread your submissions out over time.

Certain directories are considered to be “hubs” or “authorities” or both (unfortunately only the search engines know which ones, so try to cover your bases as best you can), in which case it may be used by a search engine as an indicator of the topically-relevant neighborhood that your site belongs in.

Bear in mind that toolbar PR scores are months old and can’t really be trusted. The REAL PageRank is outside of our grasp, locked up within the Googleplex.

Also bear in mind that PageRank is Google-specific. That’s not to say that you can’t use PageRank to make some inferences about the importance of a page in the eyes of Yahoo! and MSN Search. The concept of “link gain” or weighted link popularity is alive and well at Yahoo and Microsoft, they just have different ways of calculating it and names for it. At Yahoo it’s been referred to as “Web Rank” and “link flux” (a term from their days at Inktomi). I don’t know what MSN calls it. The higher the PageRank, the more useful it is as an indicator of a powerfully important site across all 3 engines. For example, I’d have little doubt that a PageRank 9 link would be an amazing link opportunity that would reap benefits across Google, Yahoo, and MSN Search.