Well our company has been successfully using Zimbra 5.0.16 for around 4 months now and it has been a great transition. We were originally on a hosted email service that did not provide us with the functionality we were looking for. It was very basic and not manageable from our end at all. We spent a lot of time testing Zimbra before going live with it in July and since then we have had very few issues. One of the more pressing issues we ran into was SPAM. With the help of these forums though I was able to tweak the settings and drastically reduce the amount of SPAM that gets to users.

Here are a few statistics from our server:
Grand Totals
------------
messages

9853 received
15195 delivered
0 forwarded
16 deferred (205 deferrals)
26 bounced
93752 rejected (86%)
0 reject warnings
0 held
0 discarded (0%)

1313m bytes received
2745m bytes delivered
1318 senders
757 sending hosts/domains
907 recipients
308 recipient hosts/domains
I am preparing for the jump to 6.0.2 shortly and I am hoping that everything goes smoothly as it has in the past for me. After that the next project will be to jump from Fedora 7 to Ubuntu 64bit. I am still pushing our company to go for the Network Edition but funds are a little tight as I am sure most of you can relate.