Fantastic. I work at an educational institution and this plugin is invaluable to me as I work with journal articles published in PDF form (the preferred way of many researchers to access peer-reviewed research these days; faster, easier, more storable, searchable, etc) -- especially since Acrobat 6.0 no longer can display PDF files in the browser window. I've already helped the head of my department install the plugin in Safari and he couldn't be happier.
I would, however, like to see the ability to view page thumbnails on the left side of the window as Acrobat permits, and the toolbar buttons should be perhaps 25-50% larger with clearer label artwork; my lab's principal investigator (who also has a copy in his Firefox installation, thanks to my high opinion of this great plugin) is constantly mixing up which is Save and which is Print.
Seems to be even faster under Firefox Aviary nightly builds than the "mainstream" releases.
And the fact that it's free for educational use (and personal use) is great. Many scientists applaud you!