Rated 4 out of 5 stars

Simple email communication and very basic calendar functionality works.
However, the integration into thunderbird is only very limited and the calendar lacks a lot of features.
Meeting invitations only work sometimes.
If you get an invitation via a mail distribution list (e.g. all-assisants@your-work-mail), the meeting invitation is not handled correctly and an error occurs if you try to accept the invitation or if you somehow try to edit the calendar event.
Moreover, only my main calendar gets synced to thunderbird. If I create more calendars, there is no possibility to add those in thunderbird. Or If you unsubscribe from one calendar, you cannot add it again.

Moreover, all the free/busy calendars from my colleges cannot be added to thunderbird.
The API apparently exists, since it is possible to see the availability if you create a dummy calendar event with one of your colleges as an attendee.
I also wrote an issue about this on their issue tracker, but no response within the last weeks.
Shared and public calendars can also not be added.

Though, all these features are available in the Evolution mail client, so it must be possible and it is completely free.

The only reason I switched to thunderbird, was the possibility to use other addons, which are not available in evolution.
So I would be very happy, if the integration with microsoft exchange will be as good as the one in evolution at some point in time.

I am not sure if I would pay again for this addon, since it is currently as good as just using IMAP for the mail communication, since I cannot really use the calendar functions right now.
Though, I would be happy to pay even more, if all functionalities are available.

Edit:
Ok, it is fair to acknowledge, that full calendar functionality is not advertised for this add-on.

This review is for a previous version of the add-on (0.8.5). 

Hi Fred, I'm sorry that you're having trouble with the calendar feature. Indeed, we would like to improve the calendar feature in the future.

Please note that calendar functionality is still in beta and not an advertized feature here on this page nor on our website. This is deliberate, because we know that our calendar feature still has some shortcomings. Could you please base your product review and rating on the features that we promised here on this page, specifically receiving and sending emails?