Co-Authors Plus is a beautifully executed plugin. The settings make sense, the co-authors UI in the edit post window is well-designed, and the fact that a custom post type is used for co-authors gives the plugin a lot of flexibility for website developers. I highly recommend this plugin.
Co-Authors Plus is the definitive solution for co-authored and guest-authored contributions to WordPress sites. I foresee it being used a lot in collaborative writing projects and in academic settings. I myself have used it in a number of academia-related projects.
It does seem that not all author-related function calls are filtered, which means that Co-Authors Plus is not ready to work out of the box with some themes that use more specific calls than the all-purpose
the_author() (e.g. the Hybrid family of themes). Perhaps this is a minor point for possible improvement in future versions. However, as it is, the plugin is already well-developed and well-maintained enough to earn five out of five stars.