I like where they are heading with event invitations, etc. Fantastic that it integrates with JSocial and Google, etc.
Ease of use
Not the easiest extension to use but it's very powerful. Sometimes, those don't go hand in hand!
Fantastic support. I even received help during their vacation time!!!! *excellent*
Lots of 1-page docs but they aren't cohesive yet. It's hard to find what you need - especially given the complexity of the extension.
Value for money
Not cheap - but if you want a good calendar solution, it's still worth it.