Zimbra community edition is Free just as much as Apache or Sendmail... What
you get in exchange (pun intended) is you support it by community or by
yourself. I have never had any issues with it, but I've only had as much as
5 people connecting. Outlook connectivity was perfect, IMAP was perfect,
caldav was perfect... And you can deploy as easily as setting up a VM in
vmware player or VirtualBox.

The things worth considering:
-What is your downtime worth? If you can take the time to fix it, then free
is cool. If email downtime is lost money, look to a hosted solution or
something with paid support (which could still be Zimbra in one of the paid
editions).
-What is your migration timeframe. If you need to have something up and
running right away, a paid package might include migration support. If you
can set it up in parallel and  test it out the wazoo, any Free package is
worth trying.
-What is your comfort level with the platform, the language it's developed
in, etc. I have had to dig into Free groupware apps to fix bugs that were
annoying me in the past, as the developers don't always have the same
priorities as you.

If you have time to plan it out and make the case, setup a VM and deploy
Zimbra, OpenXChnge, Citadel, etc and try them out, find out what you like,
get to know the community and developers. This will help you feel out what
you can expect later, and find any major tech issues before they arise with
end users. Plus it's just fun. :)

Kris Browne
kris.browne at gmail.com
612-353-6969
612-408-4431
http://www.google.com/profiles/kris.browne
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