Rated 1 out of 5 stars

FACT:
1. Anyone that says that this addon works flawlessly doesn't have to process invitations into there Google Calendar, which doesn't work, period.

2. Thunderbird Lightning is able to process Google Calendar requests out of the box, without the need of this addon, and is able to process invitations into Google Calendar, via Caldav. I urge users reading this to try Caldav first, which is the supported method.

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This add-on, while good intention, creates more confusion than it's worth, as people think that this add-on is required to access Google Calendar, and issues with this plugin and Caldav become intertwined creating more confusion on the forums and bug reporting. I urge the author to do the community a favor and remove this addon from the repository. If your going to donate, do so to the Mozilla foundation that manages Firefox and Thunderbird.

This review is for a previous version of the add-on (0.9). 

Rated 4 out of 5 stars

It's killing me... In my previous post I said that I turned to Evolution, however Evolution suks much more. It is such a shame I have to open Evolution to accept a invite into my Gmail calendar. No other features should be added until this is resolved!!! I tried adding the calendar via Caldev but still can only accept invites to local calendar. :(

This review is for a previous version of the add-on (0.9). 

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