The private beta of Spanning Sync’s full two-way iCal and Google Calendar sync app is under way.

Aaron Freedman at MacUser provides a insider’s view of how the app works and how well it works.

It is packaged as a system preference:

Spanning Sync installs not as an application, but a System Preferences panel. In it, you put in your Google Account username and password, and then select which iCal calendars to sync with which Google Calendars. You can also set at which interval to have the calendars sync (or you can just choose to sync only manually), and you can choose to sync alarms on each individual calendar. All of this can be set up very quickly, and once so, you can close System Preferences and, if automatic sync is enables, have your calendars sync in the background.

And it is working pretty well. After some initial duplication problems were sorted out, Aaron is pleased with the app’s performance:

It’s great to be able to use my Google Calendar on my PC, and use iCal at home, and then be able to sync my calendar to portable devices with iCal.