Just wanted to announce for any who may still be keeping an eye out for ZCS 6 upgrade issues that the 5.0.18 -> 6.0.1 upgrade I performed over the weekend on a CentOS 4.8 installation appears to be working great. This server was originally setup with some version of ZCS 4 (I forget which one offhand) and has been upgraded incrementally many times throughout ZCS 4, 5, and now to 6 (running ~100 users). Each time the upgrade process itself has been flawless and I want to definitely thank the Zimbra team for the excellent job they've done and instilling in me a high degree of confidence that each upgrade will go smoothly.
The only issue I've run into with 6.0.1 so far has been the small issue displaying host stats, however the patched zmrrdfetch in the bug entry (Bug 41301 – No stats on stats page) has resolved this immediately.
Good luck everyone!