When comparing KOrganizer vs Rainlendar, the Slant community recommends KOrganizer for most people. In the question“What are the best calendar apps for UNIX-like systems?” KOrganizer is ranked 5th while Rainlendar is ranked 10th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Multi-category
Pro Long-time support
Pro Integrated into KDE
It can be used by clicking on the datetime in the bottom right corner.
Pro Imports well from Google Calendar
Pro Easy and intuitive to use
Pro Integration
Your calendar data doesn't need to be stored with your computer. Data can be retrieved from Google, RTM, and even Outlook. (With the pro version removing annoying unregistered marks)
Pro Blends in with the desktop
This calendar stays out of your way, while remaining readily available to tell you what's coming up.
The Shadow skin make it look like a wallpaper calendar, but has the benefit of being interactive.
Pro Customizable
Take control of your calendar by scripting new functionality, take skins already created, or just disable all the fluff.
Rainlendar provides a platform to present your data in the way you desire.
Cons
Con Search events by text shows empty results for existing events
Con Has issues with recurring events with exceptions
Has trouble with recurring events that have exceptions to them. If recurring even has an exception to the pattern at some point KOrganizer seems to not display any of the recurring events. Thus making it useless for complex calendars. After verifying that the akonadi back end did have the full set of events, KOrganizer just does not interpret them correctly.
Con Does not connect to secure CalDAV server
Con Does not connect to Fastmail
Con Crashes even whilst trying to launch the app
Con Crashes when trying to add new event
Con Requires KDE
Con Can be unstable
The calendar crashes from time to time when entering new data.