Ken, iSynCal has not any OS X or G5 compatibility issue, I use it daily on my G5 with the latest OS X release.
Before running the first synchronizations I suggest all users to carefully read the iSynCal online help to avoid any issue, until now almost all problems reported by the users were not caused by iSynCal itself (in particular, they don't pay attention to closing iCal on the remote Mac before running a synchronization).
iSynCal
sync iCal between multiple Macs, users or with your iPod
Version: 6.0.2