Rated 5 out of 5 stars
Excellent. I've tried the addon "Google contacts" first, but ran into big problems (losing data). Zindus seems much more robust and carefully documented. It also allows you to easily use your standard Adress-Book in Thunderbird (if you want), which always is selected by default when opening the book. I am using Zindus now to sync two separate address books (private/work) using separate accounts.