You generally only see problems on mailing lists and forums, so I thought I would post a positive experience here.
I just had to migrate from a 32bit centos4 Zimbra running in an OpenVZ container to a 64bit centos5 version running in KVM.
I must have read those wiki instructions 30 times prior to moving, and I poured over these forums, but at the end of the day largely ignored most of the conflicting advice, and just doggedly followed the wiki steps one by one.
So far so good. We only had a single FOSS server with not that much data so I'm sure its a trivial setup compared to some here, but I'm glad it worked! We've managed to carry our data through upgrades and now a migration all the way from 4.5.
Thanks Zimbra for gluing together such a robust system out of all the disparate parts/projects.