IMHO, every SEO specialist should use the PR checker of the Google Toolbar and not the other checkers that are online or come with the firefox plugins

Pagerank query tools are around really long, and there has been a major change when the toolbar changed her query mechanism, having as result that a lot of tools still use the old mechanism that gives other results, with the new mechanism not resolved for a long time, as it happened i didn't follow it close enough to be sure what's the status of the reverse engineering now

there has been a lot about the pageranking query mechanism, but the only way to be sure about a pagerank is to use the google toolbar

heh i use the firefox extensions normally, but i've seen pages that gave me different results, eg. one extension gave me pr2, the other pr3

so when a big pagerank update has happened, i always check my sites with the google toolbar to see the difference with the previous update