I've been running Zimbra OSS on Mac OS X server (ppc) since the end of December.. not too long, but I upgraded from 4.0 to 4.5 without any issues, and have hit every patch level from 4.5 to 4.5.4 since.
Unfortunately I do not have the need or the money to switch to the Network edition, since I am using this at home to host email for myself, my family, and a few domains for some friends - and don't require the features of the Network Edition (the only thing that would be nice are the domain-level admins and the Apple iCal and Mobile sync), but I am happily sticking with the OSS version and have no intentions to switch to anything else - especially with Zimbra 5.0 coming up around the corner.
I've been trying to convince some other people to switch from basic Postfix/Horde/Sqwebmail/CourierIMAP setups to this - they are just stubborn
