This plugin is well designed, and I would love to use it on all my sites. My only issue is that it isn't suitable to use on a multi-page posts where we want the reader to read the pages rather than skip to other articles.
It would be great if it could be disabled for certain categories. From there, we could use a shortcode in the html to call up the box manually on the final page of the post..
I know this is pretty specific, but if you are looking for ways to improve a great plugin, there's my idea