I'm using Fruux for calendars, contacts and reminders. Very happy with it. Zero downtime since I started using it (at least one year ago, probably more).
Edit: Fruux implements CalDAV, but there are disappointingly few apps that support it. I'm stuck with Apple's Reminders. The only decent CalDAV client, 2Do, does not implement CalDAV/WebDAV correctly (they don't support "207 Multistatus"), and will not work with Fruux. When I contacted them, they expressed no interest in fixing the problem.
Edit: Fruux implements CalDAV, but there are disappointingly few apps that support it. I'm stuck with Apple's Reminders. The only decent CalDAV client, 2Do, does not implement CalDAV/WebDAV correctly (they don't support "207 Multistatus"), and will not work with Fruux. When I contacted them, they expressed no interest in fixing the problem.