Rated 4 out of 5 stars
Seems to be a cool plugin. I had a problem with using the version 0.2.3 of this plugin in combination with Firefox 3.6.x on Windows. It reported a warning about the storage in the Firebug console and the CSS Usage tab did not display any data.
The issue got resolved when I upgraded my Firefox to version 4.
Just thought I share this here in case anyone else encounters a similar problem.
Thanks, Markus - www.cross.tv/markus