I have three test installs going at the moment with some focus groups with the intention of rolling out Zimbra to replace our Squirrelmail setup. I've had some fairly good feedback from two of them with the other just getting off the ground after some teething problems and a couple of gripes which are probably more useful to share.
1. It's been mentioned before with the general consensus being that they will be incorporated at some point so I won't dwell on this but tasks are a much missed feature in Zimbra.
2. Documents. I realize they are in beta still but while they offer some really good opportunities, their implementation is a little akward it was felt. The notion of the "Notebook" organizational structure isn't really as intuitive as one might think, particularly to people who are used to a file and folder organizational method.
One major question was why can you embed a spreadsheet in a document but you cannot create a document that is exclusively a spreadsheet (Such as an Excel file is).
3. Understandably you cannot have the documents feature in the basic client but it would be good if you could at least still read them if not create them.
4. You can't share email inboxes or folders. Or at least if you can I am utterly missing where you do this. This is a big let down as a shared sales inbox is not a possibility.
5. Sharing of documents and calenders is messy. It was felt that if you share your calender with another user, it should just become visible in a users calender list. The whole inviting a person to view your calender concept meant that quite a few users where not even aware they had permission to view someone else's calender.
Here is an example of what I mean. Manager A shares his calender with the "Managers" group and sends out the invitation to share the calender to the group. The other managers receive the email and add his calender.
However after this has been done, User X, User Y and User Z get elevated to the status of managers and get put into the Managers group. However because they are not aware of Manager A's existence and vice versa, they do not get access to Managers A's calender because they never got the email and Manager A will not know to send it to them...
The same issue applies to documents and address books.
A much much better system would be if you have shared something with a group or individual, they see it by default the minute you share it or they become part of the group you have shared it with.
6. There is no year view in the calender. Unhelpful if you need an overall picture or you are plotting when to take your annual leave, training days, etc.
Overall everything else went down very well. :-)