Rated 3 out of 5 stars

Processing invitations properly into Google calendar is vital for Thunderbird. Such critical extension should be supported by the Mozilla / Lighting team, in addition to the community. Evolution accepts calendars requests properly, and to the best of my knowledge Thunderbird is the only mail client to exhibit this behavior... for many years now :( Several bugs have been logged on Google on this, and they all have been invalidated. I use Evolution now because of this, even though I prefer Thunderbird otherwise.

This review is for a previous version of the add-on (0.8). 

Agreed, this needs to be fixed. I found a new way to do so, if you want to track it see this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=684482