After a few Zimbra servers installs and Exchange migrations, I can only say that Zimbra is fantastic. There is no way way around that. The webclient & therefore Zimbra Desktop is (almost) infallible, although there are 3 issues that would make Exchange & MS-Outlook obsolete in all my cases and empower me to do many more Zimbra installations...
Issue #1 : Coloured appointments. Outlook can colour the entire appointment which is great for example doctors where reception would colour an appointment when the patient is waiting. The doctor can then with one glance see who is waiting. ZWC cannot do this and the colouring is just a thin bar to the left. Please development team have a look at this...
Issue #2 : Inline images. Although it is understandable that the multi-platform ZWC will not be able to do this, I would expect the (Windows version of) Zimbra Desktop to be able to insert images inline whilst composing a new mail message. Just paste your images inline like almost all but Zimbra desktop can do.
Issue #3 : Default mail handler. If you want to quickly send a text or spreadsheet document from within MS-Office or OpenOffice(file/send-to), or just send an attachment from Windows explorer (Right-click, Send-to), you cannot do it with ZWC (which I can understand) nor Zimbra Desktop(frowned upon).
Although Zimbra is great, the ZWC & ZD is easy to use and fast (compared to you-know-who) and server is amazing, these 3 issues is the most deabilitating when trying to convince companies to switch to Zimbra and I've been unable to convince clients to make the switch to Zimbra, solely based on these 3 issues.
It might not be 'user' related, but in the companies I consult to, their IT personell is also like 'users' to me. So from a server perspective, the most limiting lack-of features is black/white listing or just general administration regarding spam control. If Zimbra can 'borrow' a few features from 'mailscanner', it would be the end of all non-Zimbra mail servers, as you cannot compete with perfection!
I hope my ramblings do not fall on deaf ears and if you agree, give a thumbs up.