The initial sync of my Gmail account took about an hour from start to finish. I just left it alone while it downloaded my mail. I didn't have much mail in my ZCS account, so that sync was almost instantaneous.
The e-mail composer has all the features I expect from a modern client. You can compose in HTML or plain-text format, and in HTML mode, you have all of the usual control over fonts, lists, colors, text size and so on.
The e-mail viewing interface has the familiar three-pane view that most clients default to, and the HTML e-mail messages I used for testing rendered the same as in other clients.
On the whole, using Zimbra Desktop is quite plain. It feels like what I'm used to with standard “fat” desktop mail clients. The interface is easy to navigate around, and things are generally about where I expect them to be, and when I wanted more information, the integrated help system was very useful.
One of the biggest differences between Zimbra Desktop and a standard client is that there aren't the normal File, Edit, View, Search (and so on) drop-down menus arranged into a nice menu bar. This is not a limitation as such, it's just different. In normal, everyday use, I didn't miss them.
One of my favorite features in Zimbra Desktop (which is also one of my favorite things about Gmail) is the extensive availability of keyboard shortcuts. In fact, many of the key combinations are similar, if not the same, so I quickly felt at home. This is something fat clients could learn from their browser-based cousins—easy keyboard shortcuts, such as pressing J to move down in the list of messages and K to move up, are great time-savers. The complete list of shortcuts is available under Preferences→Shortcuts.
The calendaring component of Zimbra Desktop is very nice. It's much better, in my opinion, than Google Calendar, if not quite as good as some of the other desktop calendar apps I have used.
You can create appointments by a click and drag on the calendar, and you can move appointments by selecting them with the mouse and moving them where you want. You can create new calendars simply by clicking the New Calendar button.
The shared calendars and scheduling features work only with ZCS accounts, so keep that in mind if you are thinking of using Zimbra Desktop in a multi-user setting. If you are using Zimbra Desktop with an e-mail account other than ZCS, the calendar works like a standard desktop calendar. You also can subscribe to shared ical calendars.