We have many users who have upgraded to Sierra with success. It still requires xQuartz but you can use the latest version (2.7.11) rather than the previous version (2.7.9) that was required for 5.3/85. We have run into an issue opening the help that should be resolved in the upcoming service pack for 5.4/8.6:
http://www.harrisgeospati...-on-MacOS-10125.aspx