Just installed Zimbra last night without a hitch, and an impressive feature set to boot.
Some background: What I really need is an appointment scheduler with contacts (I don't mind the email but its not what I need). So I'm really not sure if Zimbra is a good fit but the interface is so slick I want to use it

and the SOAP interface is great.
Couple of things I need to change would be:
1) Adding custom fields to the Contacts (IM interfaces, primary phone) and some checkboxes (use this IM server for instance, appointment notification)
2) Do not automatically send email appointment invites (I want to control that and the method).
3) Be able to add contacts to an appointment that do not have an email address, but that have other information (from what I read this looks like it maybe a problem trying to do). I know you can kludge it with a invalid email address but for my target audiance things need to be very simple and intuitive.
Things I would like to have:
1) On logon, calendar shows up first not email
2) When scheduling an appointment be able to select attendies right there instead of going to advanced.
3) Event driven third party hook. So that I could listen (so to speak) for Create Update Delete on appointments without polling.
I haven't really dug into the code much but any idea on how difficult it would be to implement some of these changes.
Any advice is appreciated.