Rated 1 out of 5 stars

ok, it duplicated all my contacts in google, then double duplcated all my contacts in tbird.
Then i fixed google, hit sync and it disables sync.
So I hit RESET, which DELETES ALL MY CONTACTS.
If you do this, search through all the advanced dialogs till you find the temporarily disable sync checkbox and uncheck it, its invisible and warning-less, but if you hit reset with it on it copes an empty contact list to your contact list, deleting everything.

So once its up and running, it appear to work, but good luck if you already have contacts both places.

This review is for a previous version of the add-on (0.3.3). 

There is a long explanation of what "Resetting" an address book does and gContactSync warns you before you do it. If you don't read dialogs before clicking buttons at times you might get a surprise such as this one.

It DOES NOT ever delete all your contacts; it removes them all from either TB or Google depending on which type of reset you selected then after you restart Thunderbird it will pull the contacts from the opposition location. If gContactSync is going to permanently delete more than 5 contacts (the number is customizable) then it will ask your permission first.

You should start from an empty address book, not one that was already synchronized with or imported from Google. gContactSync used to detect and merge duplicates but it was slow and people complained so I removed that feature.

Josh