Rated 1 out of 5 stars

1. Don't sync merged contacts in webpage of Google mail
explanaiton
1.1. select 2 contacts in webpage and "merge" them
1.2. sync manualy - or let the plugin to sync, and the contacts in Local addrss book is unchanged

- if you don't merge contacts, you open one contact and copy necessary data from it and paste into the second one (the good one) save second one and delete first one (bad one) the sync is correct - that means it don't understand the "merged" log on gmail server

2. is not able to make any difference by groups

3. I tried several times to use it but every time I I use it it mess up my gmail contacts. (if I change something on web page the sync is made almost only manual), if I change something in thunderbird very often that changed don't apear on gmail account

This review is for a previous version of the add-on (0.8.13). 

1. I am not able to reproduce the problem you describe re: merging, please file a bug report: http://zindus.com/i/reporting-bugs

2. if you make a change in TB and sync, the addon updates Google's servers. If you want to see the change immediately in your browser you must force a cache refresh - either click "reload" or log out and log in again.