Absolutely fantastic plugin for better page loading speeds.
I've tried it out and things were performing great ... until I quickly noticed that the site's comments were missing all gravatar images, for some reason. You can see the example I took from http://www.geekandjock.com/arguments-relationships-are-like-cats-and-boxes in the screenshot snippet below:
I needed to deactivate the plugin but hopefully you might be able to make some sense from running Firebug or something on the site's gravatar areas for some insight.
Again, great plugin in what it's trying to do - Google will love site owners more with it too :)