Rated 4 out of 5 stars

Nice work. But lacks one crucial feature i.m.o.. Please add possibility to order items vertically. For instance : i add a number 2 in front of the title of an existing event and the event jumps to it's new position, under event with title beginning with number 1 etc. Google calendar supports that too. So that would be a nice way to order the items, which works cross platform. Mobile too. Vertical ordering is crucial, for effective long term projectmanagement I need to create subtasks. I'm sure a lot of project managers would like this feature.

This review is for a previous version of the add-on (1.0.0.0b5). 

Interesting suggestion. I'll look into supporting this "override" ordering, it should be relatively easy to implement.

Parent tasks/subtasks functionality is on my wish list. I'm currently working on another extension that uses an SQLite database to store custom event/task information. I will use a similar approach for this functionality, as well as for task/event dependencies. But it could be a while before this happens.

You could use multiple calendars and the "group by calendar" feature to roughly approximate subtasks - but this would only give one level of parent (calendar) and children (events/tasks).