Rated 2 out of 5 stars

In an earlier review I stated that the sync to Google did not work. That is, synced the contact so could be found with a search, but not in My Contacts. I subsequently found they were in More Contacts, and had to be manually moved to My Contacts. Then they would sync to Android. But, the program does wipe out Groups, every now and then, questions on the Forum were not responded to, and I am afraid to rely on it,

This review is for a previous version of the add-on (2.0.10). 

I think I found your post, and it was in the incorrect group (Other Add-ons instead of gContactSync) so I missed it. You probably chose to sync All Contacts And Groups. When you do this and create a new contact you must copy it into the My Contacts list in Thunderbird if you want it to show up in your default contacts list in Google. This has been addressed several times in the forums. There is an outstanding feature request to automatically add new contacts to a default group that will be in version 3 or 4.

If you only care about My Contacts, then I recommend only synchronizing that group.