This is a great plugin. However, I encountered a problem when using it with a responsive template from joomlashine. While scrolling down the page, and the "back to top" image was to the right side of the responsive content, the "back to top" link didn't work, even if it showed up properly. The solution was to change the "back to top" z-index value to a greater value than the one of the content. Thank you for this wonderful plugin.
The latest versions of the plugin have an option for automatic z-Index. This was a common problem, so I wrote some JS that could overcome it.