I have khal (calendar), khard (vcards) and todoman (todo on caldav) installed and use them occasionally. I mostly use my phone and thunderbird to manage contacts and calendars - so they don't find regular use. However, they all are really nice curses/cli tools if you live on the terminal. I don't have any complaints when I do use them. The biggest attraction though is vdirsyncer, a service used to synchronize CalDAV and CardDAV to a directory. Khal and others interface only with this directory and not directly with the server. Vdirsyncer allows you to easily backup and migrate your calendars and contacts between different servers. This is a huge relief if you consider how opaque and locked-in most calendar/contacts services are.