This was useful to me: I've had trouble managing vcard with mutt, and without mutt I'd probably have to give up email. I began using caldav for a similar reason: I'm on Android and my wife is on iOS, and it annoyed me that Android tries desperately to standardize you on Gmail and addressbook, while iOS insists you use iCloud. I like maintaining a degree as independence.

I'm not as clever as the individual who wrote this useful blog, but in my case I set up an account with Fruux, who offers vcard and vcal as a service. It syncs perfectly across several OSes (on Windows I used eMclient, and on Linux as I mentioned I've struggled). Anyone who wants to avoid setting up custom servers, Fruux handles that part easily.

This afternoon I'll try to see if I can get vdirsyncer and khard working. I really wish the likes of thunderbird and kmail offered better carddav and caldav support. Evolution does, but I'm not a huge fan of Evolution (and not sure why).