Rated 5 out of 5 stars
I have noticed that using this app makes existing address books corrupt. Dragging a contact from one address book to another makes the address books corrupt, and makes other contacts disappear. Maybe for just one address book ok, but if you have multiple address books, be careful.
Edit 5-12-2017: this week I noticed the same behaviour without the add-on, and I created a new profile within Thunderbird. I realized that I have used the profile for years now, and old remnants from the past may have caused the address book behaviour. So far everyhting works fine now :)
The address book in Thunderbird is extremely buggy and fragile. I try to work around those problems in gContactSync where I can, but sometimes changes in Thunderbird make my workarounds no longer work.
Which versions of Thunderbird and gContactSync were you using? There was a drag and drop problem introduced by a change to TB that wasn't compatible with my address book modifications that I fixed here: https://github.com/jdgeenen/gcontactsync/issues/109
Edit: I highly recommend moving contacts within an address book from Google's interface. Moving them between accounts can be done with Google's export and import. This is far more stable than using Thunderbird.