Congrats to the Zimbra team. Just upgraded ZCS 5.0.18 Ubuntu LTS 6 x64 to Zimbra 6. No problems at all, went very smoothly and did not take that long either - 20 minutes.
The service logger keeps on stopping and I have to manually restart it (stops again <24 hrs later). That's the only 'buggy' thing I've noticed so far.
Will be doing multiple SLES10, RHEL4 , RHEL5, and Ubuntu LTS 8 servers next week. So will report on how that goes too
Zimbra 6 is a great release!