I just performed an update from a Zimbra NE 5.x.x on a 32 bits dappers to the latest Zimbra NE 6.x.x on a 64 bits hardy server, and I was amazed at how easy the migration path was to follow. I had a few problems, but non were related to ZCS. I was also extremely pleased to see how well documented zimbra is !
So I have no questions, I just want to thank the zimbra team for their awesomeness. You made this system administrator's job a lot easier, and I will continue to recommend your product to my current, as well as my future, clients.