I found one thing my users do not like about Zimbra. I have this deployed in a drug treatment site and they use the calendar for client appointments using the web interface. There are many times that they need to look back to see all of a clients appointments. The search works fine for their needs but they have to go month by month.
They would live to see an option to view all results in a list form like Outlook used to do. I would not think this would be too hard to add to a future update.