Today we have performed an upgrade from 4.5.11 NE to 5.0.2 NE in a CentOS 4.6 x64.
The upgrade finished successfully with only a minor issue with unicode_start process problem, solved by removing .UTF-8 strings from /etc/sysconfig/i18n
Unfortunately, this upgrade was done over a restored backup of our production environment, so we will repeat this upgrade this Sunday, this time in our real server.
Anyway, the upgrade has been really easy and we have not found major problems, although we had to adapt some customizations to new version 5.0.
Great job, Zimbra team!