Rated 5 out of 5 stars

CardBook is the missing link to finally tie TB straight in with cloud and phone, fantastic!!
* Reliable syncing to NextCloud: While inverseSOGO gave me lots of trouble, CardBook worked out of the box.
* Standards compliant: No information is lost when syncing with your phone. In contrast, the standard AB has a second address line which is ignored upon sync.
* Convenient: Full text search, groups are accessible with one click and map into android [in DavDroid set per-card-categorie]
* Allows calling Skype without Telify AddOn [under preferences-internet communication add Phone Code:skype Protocol:skype]
* Make sure to configure in Thunderbird preferences, Display tab, uncheck the last option : "Show only display name for people in my address book". Otherwise it was very laggy for me [see SOLVED - 21.4 and 21.5 makes Thunderbird extremely laggy]
* Fantastic support, Philippe is very responsive!

This review is for a previous version of the add-on (21.5). 

Rated 4 out of 5 stars

I can finally sync Ubuntu 16.04 running "radicale" with Android CardDAC-sync, fantastic, thank You VERY much!

But I have two issues:
1) CardBook restricts address auto-completion to CardBook; I did not find a way to include my local Addressbook when adding emails.
2) Editing and management in CardBook is not as convenient as the built in Address Book. While I understand that the VCARD standard limits the field, editing, searching and grouping in CardBook is much more difficult.

If CardBook could use the already existing AddressBook implementation, and just add another Book, I think this would be ideal.

This review is for a previous version of the add-on (11.2). 

1/ it is possible : in CardBook preferences, in the first tab uncheck the value of "CardBook as unique source"
2/ I will try to improve this point