Rated 5 out of 5 stars
With all of the disinformation flying about these comments, it must be hard for the uninitiated to make a rational determination as to the value of this add-on.
There are four independent yet interdependent components to consider when installing, using, and upgrading this add-on:
1. Thunderbird or SeaMonkey (the base component)
2. Lightning (the calendar component)
3. Provider for Google Calendar (this add-on)
4. Google Calendar (the data component)
When upgrading, it is extremely important to remember that changes to Lightning and/or Thunderbird (or SeaMonkey) will very likely have some impact on the behavior of Provider for Google Calendar. If available, you should upgrade your build of the Provider at the same time as you upgrade Lightning.
A good rule of thumb when upgrading any of these components is to disable *all* remote calendars before the upgrade, then upgrade Thunderbird (or SeaMonkey), followed by Lightning, and finally Provider for Google Calendar. Once all that's done, review the properties of your remote calendar(s) to be sure that nothing broke in your profile, and only then, re-enable your remote calendars.
CalDAV support for Google calendars is great but lacks some functionality (notifications via SMS, for example - at least, this has been the case; it is likely to change in the future, but that is entirely up to Google). Whatever changes Google may make to various external calendar interfaces are beyond the control of this add-on's developer and anyone else outside of Google, so please don't blame this add-on (or it's developer) for such happenings.
It works for the vast majority of us using it, and the author is active in development, responsive to bug reports, and approachable. Beyond that, the price is truly an exceptional value (FREE), which is something often overlooked in the open source community.
As with all things in life, your personal experience may vary, and no two people share the same data conditions at the same time. This add-on works very, very well for me, and I highly recommend it.