My main problem with the iSync connector is that I suspect that it's responsible, or partly responsible, for massive duplication of contacts in Zimbra. Can't say for sure; iPhone ActiveSync may be another factor.
That said, with regard to synching the GAL, you may or may not be aware that Mac Mail will autocomplete based on the GAL if you have LDAP configured in Address Book. iCal 4 will also autocomplete based on the GAL, but only if you have a newer version of Zimbra. (The feature was added as part of
https://bugzilla.zimbra.com/show_bug.cgi?id=21489. 5.0.18 is the first release that has it.)