Great add-on, apparently unecessary now Rated 5 out of 5 stars
I have used this add-on for months and was very happy with it. Suddenly it stopped working and when I checked my add-ons, Thunderbird was saying 7.0.1. was not compatible with the add-on. While googling for a solution I found we can add a Google calendar to Thunderbird as a CalDAV (File>New>Calendar, On the network, ...). I tried and it works in both directions just fine.
I am using 7.0.1 on Ubuntu 11.10