Great add-on (+ test results with the TB/Lightning built-in caldav support) Rated 5 out of 5 stars
This add-on has worked pretty well for me since many years, up until the recent freeze/hang issue with TB15/Lightning 1.7. While I'm sure it will be resolved promptly, this has been the occasion for me to try the built-in caldav support in TB/Lightning. So far, so good, it's been 5 days that I've been using my various Google calendars on TB/Lightning (on Windows & Linux) with caldav support (so without the Google calendar provider add-on) and everything is working fine!
For more info, see these instructions: http://support.google.com/calendar/bin/answer.py?hl=en&answer=99358#sunbird
It's great to have this kind of alternative built-in support. Of course, the Google calendar provider add-on still rocks! (...and it should definitively be implemented directly in Lightning IMHO, as it is a little bit more easier to configure than caldav.)