my upgrade on Ubuntu 8.04 from 5.0.18 NE to 6.0.2 was successful without any problems.
On 2 steps of the processI was suprised, but could easily figure out what to do:
1. Installer of 6.0.2 indicates prior to installation that sysstat is missing. Solution: cancel process. aptitude install sysstat. install again - now the installer finds it and everything is fine.
2. The installer suddenly presents a text based menu. Reason for that: there is a new configuration option that you have to set. You just enter the number of the entry with leading * symbols. You have to choose if you want to enable update notifiacations (reminds you when there are new Zimbra versions available). I chose yes, then the process continued.