MOLAMOLA Google Calendars synced to iCal are Read Only when you sync them to the iPhone which means you can not enter / change anything using your iPhone.
So in case you use an iPhone and want to use the free google calender instead of a terribly expensive mobile me subscription to get a Mac/iphone and Web based calendar solution, BusySync will pay of after just 3 months (compared to subscribing to mobile me).
So it makes perfect sense to me, at least as long as Apple does'nt provide a way to make google calender items writable on the iPhone.