Hopefully our experiences will help others.
We upgraded from NE 5.0.19 to NE 6.0.2 on the Mac OSX 10.5.8.
For the most part the upgrade went smoothly, however we experienced the following issues:
1. SSL certificates need to be re-installed manually as detailed here.
2. All the icons for the advanced web client and the admin console are missing (unresolved). However they appear in the standard client.
3. The administrative "Downloads" link for add-ons such as the Outlook plugin are all missing.
Besides these issues the installation is successful. Issue #2 is most pressing for us since the majority of our clients use, and prefer, the web client.
Hope this helps.