The Art of Advanced Link Building: "Daniel Dulitz, a software engineer at Google, spoke candidly about PageRank and technical issues currently surrounding the use of the Google Toolbar, saying '0/10 is inaccurate at many times, and because data isn't on a consistent update schedule, it is unreliable, and a poor measure of content value.'
Of the Google toolbar, Greg Boser strongly says 'uninstall it!' Even though the toolbar offers a basic gauge of PageRank (and therefore link popularity), it's simply not worth the obsession if rankings are holding fairly steady and traffic has not dropped off.
Google emphasizes that by placing a link on a site, you are associating your site with another, thereby lending credibility to the quality of content. However, certain linking situations can come with unwanted baggage, such as linking to trademark names while making false claims about the company or holder of the trademark.
But if it's fair comparison, it might not be illegal - so check with a lawyer. One trademark owner in the audience said you can take advantage of sites that mention your trademark or brand name fairly, and require a link to the official website in exchange.
When in doubt, it pays to follow Danny Sullivan's golden rules of link building"