I ran the full 188.8.131.52 upgrade today (6.0.6_GA_2330.RHEL5_64_20100505193959) and the upgrade from 5.0.13 seems to have completed without issue. I made multiple backups and thoroughly tested the upgrade on my disaster recovery platform first which ran fine. As a result, I was pretty confident it would go off without a hitch but a major upgrade on a production machine is always a bit unnerving. Hopefully, end-user adoption will be quick. Kudos to the Zimbra team for the smooth upgrade process.