Outlook is the application that people can't get away from. The server behind it is largely meaningless as long as you can get the data. Even Microsoft finally is recognizing this with it's recent moves to put applications into the cloud.
Google Calendar Sync allows you to sync events between Google Calendar and Microsoft Outlook Calendar. You'll be able to determine the direction of information flow, as well as the sync frequency. Staying on top of your Google Calendar and Microsoft Outlook Calendar events has never been easier!
Keep in mind that it's not possible to sync events on secondary calendars at this time. Google Calendar Sync will only sync events from your primary Google Calendar and your default Microsoft Outlook calendar.
My company is a Zimbra shop and we've been happy with the functionality and uptime. But, we've been endlessly annoyed by the missing mobile ability. Dare we look at GAPE again?