Works great. You only have to set it up once (unless you’re adding or removing new lists/calendars), and then it seamlessly syncs your address books, calendars and tasks between your CardDAV or CalDAV server and your Android device, and you can use them with any app on your phone that uses the system calendars and contacts.

I can use my Nextcloud contacts with K-9, GMail, the built-in Contacts app or Fossify Contacts, and even for phone calls. OpenTasks works great for my to-do list. And I can use my Nextcloud Calendars with anything from Simple Fossify Calendar up through the Google Calendar app, and it all syncs up and down.

Note: With Nextcloud, syncing may be faster with an app-specific password.

Online at DAVx⁵. Available from Google Play, F-Droid.