Google's internal rankings are obviously pretty important, but given that they only update toolbar PR 2 or 3 times a year, it seems it isn't a very accurate metric. Having said that, I think a high PR is a useful indicator of the value of the links on a site when considered along with other factors - such as SERPs, quality of site, amount of content, regularity of updates, site age etc.
A lot of people value it and it seems that a high PR means good money for links.
Interested in quality, relevant link exchanges and deals, especially with UK sites.