I agree with what @Mittineague says. The only links really worth having are those which you earn, rather than those which you place yourself. Links from relevant sites will always be more valuable - and more likely to bring relevant traffic to your site, too.
Google has long been telling webmasters not to obsess over PR, and because of that, has stopped publishing regular public updates of PR. So a site may show low PR, or have no available information, and yet be regarded highly by Google. By concentrating too much on PR, you may run the risk of missing quality sites.
Does that help?