You have a list of CalDAV clients at http://caldav.calconnect.org/implementations/clients.html
While we don't certify any particular client at the moment, both "aCal for Android" and "CalDAV-Sync" participated in Calconnect interop testing events, and hence were tested against our server.
Shortly we deployed a App called CalendarSync to our Brazil devision with about 30 Users. All of them got new android devices and are using this app to sync with our oracle servers using CalDAV. So far every thing works well. Bought the app directly from the dev to get a discount, and some free test licenses to check your setup. (You can find some contact informations at this page:http://ntbab.dyndns.org/apache2-default/seite/icalparseandroid.html) But the app is also available at the android play store.
because in CommSuite davserver exists currently no carddav support, I have wrote a script to sync CommSuite (private) Addressbooks in a davical davserver (witch support carddav).
It use the carddav protocol, so other carddav server should also useable.