Rated 4 out of 5 stars

I used this addon for a long time and it recently added task synchronization support too. But now it doesn't allow me to directly add my google calendar using "New Calendar -> On the Network -> Google Calendar", because it conflicts with the other addon "Thunderbirthday"!
There are 2 solutions:
1. Disable thunderbirthday, Add your calendars, Reenable Thunderbirthday when finished.
or
2.What Shelku said:
2.1. New Network Calendar
2.2. Format = iCalendar (ICS), Location = Private address in ICAL format (NOT XML FORMAT)
2.3. Restart TB
2.4. This addon will automatically detect your readonly ICS calendar and will ask if you want to upgrade it to a r/w google calendar

This review is for a previous version of the add-on (1.0.2). 

Rated 4 out of 5 stars

Also TB has Google Calendar integration by default using CalDAV protocol but after searching through web I found out that this addon has some little more features and is easier to setup.
Plus this addon provided Google Calendar support before Google support CalDAV itself.
I gave it 4 star because it doesn't support Google Tasks syncing with Lightnings' tasks.

This review is for a previous version of the add-on (0.18).