Rated 3 out of 5 stars

I am not yet ready to do a full review, as I just started using the add-on. It does seem to be working well, but two things prevent me from going further.

First, the link to documentation still points to a Google Code site. I believe Google has discontinued this service, or perhaps restricts access. Either way, I cannot read the documentation and suggest moving it to a more public place.

Second, I have two Google calendars with names that start with "Personal." "Personal - Long Term" has birthdays, anniversaries, dates for memorials for dead friends & relatives, etc. "Personal - Short Term" has doctor appointments, scheduled car services, meetings with friends, parties, etc. But the drop-down menu in the add-on only shows one choice, "Personal." Since the calendar names are legal for Google Calendar, this seems to be a bug that should be fixed.

I'd also like to be able to specify a time for a task. (E.g., to make a phone call at 11 AM.)

P.S. I would have written to the developer directly, but I cannot find an email address.

P.P.S. Regarding XtremeVision's comment about auto-sync and Tomasz's reply, I suggest always syncing when a new task is added and graying out the sync icon until the user changes a task's content. (E.g., the way LibreOffice grays out the save button after a file has been saved.) Since this is a two-way sync, there might also be an option to activate the icon/button at specific intervals.

This review is for a previous version of the add-on (0.5.2). 

The Google Code site never hosted any documentation, just the source code (which is available right here on the Mozilla add-ons site) and an issue tracker, which mostly fell into disuse long before Google Code was shut down, as I stopped actively maintaining the add-on. For the record, I am unaware of any forks, and no-one ever expressed interest in taking over the original project either, so any updates or bug fixes are unlikely.