Rated 4 out of 5 stars
Excellent AddOn and must have for Nextcloud, Zentyal and SOgo.
Could be five stars but looses color in calenders in the windows version on Zentyal's SoGo - all calendars are grey (color choosen in Linux is kept with Nextcloud).
Update: Unfortunately one can not change the color of Zentyal SOGo. If you connect the calendar natively using lightning you can overwrite the color from TB side.
Due to popular request, the selected color in lightning will be transmitted to the server.