I'm a long time user of the #Mutt mail client. Calendar is a little problem. I use my own #DaviCal server (#caldav and #carddav ). Most of the linux clients sucks - Evolution has some trouble with carddav, KOrganizer and the Akonadi thing are not working as expected an Lightning is horrible slow.
@Simon L'nu Yep, Mozilla Lightning. It could be a really good caldav client as it supports some of the most important features. But it is really slow on large calendars. carddav support is also missing.