I've been using Zimbra for about 6-8months for my previous small business and what is now a personal/home mail server. I'm not a Linux guru but I'm not exactly a slouch either.
I've recently installed the Open Source version for a friend's wife's office. She has 6 people who work there who were all using Outlook POPing to Gmail accounts.
She's beyond satisfied with the powerful features Zimbra gives her and her employees. Being such a small company she wasn't expecting to have all the sharing features Zimbra offers. Three of the six employees need to use a shared calendar & contacts regularly.
I installed Ubuntu 6.06LTS on one of their spare machines and then installed the latest Open Source version without a hitch. I started the install about 10am and she was using the system by 1pm. I had already done the legwork of securing a domain name and setting up the DNS.
The kicker is that her husband has his own company of several hundred employees. He was looking at moving from GroupWise to Exchange but is now going to have his IT staff look into Zimbra as an alternative.
The only thing that she was disappointed by is that her and her employees cannot sync their phone's with Zimbra. I explained that is a NE feature and laid out the cost, which is well beyond her budget.
Other than that, you have another satisfied user and, potentially, another good size customer!