On the whole, I was impressed with Zimbra Desktop. But, there were some things I really didn't like. As Zimbra Desktop is in beta, take these criticisms with a big grain of salt, because they might have disappeared by the time you read this.
Setting Zimbra Desktop to connect to my Gmail account was an easy process, but using it was not so easy. When connecting to Gmail via IMAP, your Gmail labels show up as folders, and the first time you click on any folder other than the Inbox you get a “This folder is currently not syncing. Click here to sync this folder” message. This is silly, because if I did not want to view the messages in the folder, I would not have clicked on the folder. The very action of me clicking on the folder indicates that I want to sync the folder, so there is no need to ask me. In addition to this, after you click on the link, you are sent back to the Inbox for some reason. Clicking on the folder again finally gets you the folder you wanted. Three clicks just to view one folder seems excessive, and the effort involved in getting my entire Gmail tree syncing was unnecessary. I will grant that I have a lot of labels, and thus, a lot of folders, but Zimbra Desktop could have been smarter with its default behavior.
Another annoyance is that in the Zimbra Desktop interface you can view only one account at a time. Because of this, dragging e-mail from one account to another one is impossible, as far as I can tell.
Each account also has its own calendar and contact list. I have a work e-mail and a few personal e-mail accounts, and both work and home calendars. In Zimbra Desktop, there does not appear to be a way to view all contacts from all accounts at the same time. Ditto for calendars.
Another minor quibble about the calendar is this: because I'm using Zimbra Desktop with my Gmail account, when I saw the calendar, I thought to myself, “hey, this probably will sync in both directions to my Google Calendar”. It doesn't. Zimbra Desktop can read and show your Google Calendar by subscribing to the ical link you can get from Google Calendar, but it is read-only. The writeable calendar is only in Zimbra Desktop. I suppose I can't complain too much, as Zimbra never said the calendar would sync with Google Calendar, but I'm still sad it doesn't. Maybe in a future version?
On the topic of missing functionality, I was not able to get the touted off-line capabilities working with either Gmail or the ZCS server I set up for testing. I tested the feature simply by unplugging my computer from the network and then launching Zimbra Desktop. Instead of showing my e-mail messages, I received a message stating that the document could not be displayed in front of a blank window. This should improve with future releases and may well be fixed by the time this article is published.
There also is a dæmon that must be running for Zimbra Desktop to start. If it is not running, you'll get an error message.
This dæmon lives at zimbra/zdesktop/zdesktop and must be launched with a single start argument. The installer does tell you about it, but it does not set it to start automatically, so in order to make sure it was always running when I was logged in, I created a Startup Program entry for it. This is a small annoyance, and one that hopefully will be automated in a future version of the installer.
One final thing to be aware of when using Zimbra Desktop with Gmail that I found is annoying is that if an e-mail does not have a label assigned to it and it is not in the Inbox, you won't be able to see it. So if you plan on using this with Gmail, label everything.