I just set up the isync connector for our company (5.0.16).
All it does is automate the creation of a caldav entry in iCal, which I had already done myself and was working (apart from the known issue involving osx 10.6 and not being able to update calendars. I'm harping on the sysadmin to apply the latest patches for that...)
Not only does it not sync the GAL (which is probably the second most important part, after calendar) but it makes no effort to separate the synced address books in Zimbra to Address Book on the mac. It wiped out my existing contact groups on first sync, and then combined the whole thing into one big shapeless and disorganized mass.
I can see from searching the forums that these problems have been around for a very long time (The earliest message I found regarding the groups issues was from sept '08). The GAL workaround is flatout unacceptable, especially since there is no way at all to segregate the GAL contacts from other contacts without extensive manual labour.
These are things that have been requested by countless users, and at least as far as the GAL problem is concerned, Zimbra has flat out said that it will not fix this problem.
I would very much like an explanation as to Zimbra's refusal to fix these problems.