Rated 3 out of 5 stars

This add-on works, but for some reason it only syncs two email addresses, this is not well-indicated in the app, and you cannot change this in the settings. Not very useful for business users.

This review is for a previous version of the add-on (2.0.7). 

This is a limitation of Thunderbird, not gContactSync. It actually syncs 4 e-mail addresses: the two included in Thunderbird and two additional e-mail addresses that gContactSync adds. When I started working on this add-on there was a feature request for TB to add support for n phone numbers, e-mail addresses, etc. but it was never finished.