Highest functionality of any joomla calendar extension. ACL, Integration (Google, iCal, others), Social Networks, Attendance and Payment, ..
Ease of use
Very clean extension. I tried other popular calendar extensions and it was never as satisfying and simple as with dp calendar.
Best support I experienced in the joomla world. Responses within a day and always polite and target-aimed until your problem is solved.
It is very detailed and helps you customize the calendar to your very needs. If you have problems understanding it, the support will help!
Value for money
I payed for the professional version and am very pleased with it. It grants high functionality and access to the support area. Worth it !