I use SEO Quake, a Firefox plugin to gather information about a site. My configuration shows me a lot of relevant data when visiting a site. Instantly I do see Google PR, Alexa ranking, inbound links on Yahoo and Google, amount of indexed pages on Yahoo, Bing and Google, domain age, keyword density and so on. I can only recommend this nice plugin when looking at other sites in search for a trade:
http://www.seoquake.com/
Personally, I prefer to look at content of a site, inbound links and Alexa ranking, Google PR is just a secondary indicator for me.