Rated 5 out of 5 stars

Obviously, this is still early days. But I have been using this plug in with three separate Tbird addressbooks totaling 10K contacts and 50 labels/distribution lists. And I'm using it in full read-write mode.
Marco has been fabulous in his responsiveness when I report a bug.
Note that for really big addressbooks, the sync may take 5 minutes or more in full read/write mode.

This review is for a previous version of the add-on (0.1.5). 

Thanks for Your report.
Read-write mode is only partially working though, as there are still some bugs and the addressbook structures are different between Google and Thunderbird. So I'd suggest to stick to the read-only mode.
And yes, the slowness is another major drawback, which will hopefully be overcome once TB91 and the new TbSync are out.

Thanks,
MZ