Now that I have tried it out, I am impressed with how well it works. I was easily able to synchronize a calendar in my Google Calendar account with a calendar in iCal. In fact, this was a Google Apps for Your Domain account, so there doesn't appear to be any problem working with these, as opposed to a regular Google Calendar account.
One suggestion that I have is to add the ability to create a calendar in iCal from the Spanning Sync interface. This would be very helpful if you have a lot of accounts in Google Calendar, and want to synchronize all of them.
I don't think that I will be using Spanning Sync personally. I am finding the calendar sharing and event invitation capabilities so much better in Google Calendar than they are in iCal. Also, I can modify the calendars on Google, even when I am not on my Mac. The only reason that I want to keep things synchronized with iCal is so I can use iSync to synchronize the data with my phone.
This is really easy to do with just subscribing to the private ics feed in iCal