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Pro Integrates with the desktop in order to provide notifications
California offers desktop notification on most major DEs in Linux, which is great to know when events are happening while not having to have the app open in the foreground.
Pro Events can be added using natural language
The application allows natural language input for events. For example, typing in "Lunch with Stuart Kearney at 2 pm" will set the appointment.
Con Currently not in active development
Yorba, the group that developed California, is no longer actively developing software.
While Geary (Yorba's mail client) has been adopted by the elementary teams as "Pantheon Mail", both elementary and Gnome have their own calendar projects and it is unlikely that either of them will contribute to California in the future.
That being said, California uses evolution data server for all backend/sync/storage related tasks, which should hopefully make it possible to use it even if 3rd party vendors change their APIs/protocols etc. As long as EDS does not introduce backward incompatible changes or any major California bugs occur (it has been working fine for me so far), the current state of California should remain usable for quite a while.
Con Currently no drag and drop support
There is no way to drag events in the app for easy switching, it all has to be done manually which can be a lot of work to move many things around.