Mature, complete product with lackluster support
I used Zimbra as a freelance community strategist/manager for a client from 2014-2017. I've completed the organizational demographics fields for the organization I was contracted with, not my consultancy. My client used the on-premise version of the Zimbra platform.
The good: This product is very mature and full-featured. I think my favorite features were the configurations for the news feed and the widgets. The news feed was ridiculously configurable. Who should see what kind of content in the feed? What kinds of content should be in (or excluded from) the feed? Whose content should be visible by user type X? Those are only a few questions to ask yourself when setting up the news feed. The widget studio was also very cool. Nearly infinite possibilities on widgets. My client spent extra time and money on the design phase and, as a result, the user interface was beautiful and easy to understand.
The bad: I'm uncertain how much of my negative feedback is due to the server environment my client used, and how much is due to the Zimbra product, so take all of this with a grain of salt. I found the page editing process to be unnecessarily complicated and inconsistent. There's a feature that allows the community admin to apply page changes to just the admin, for everyone on that page, and for all pages on the community website. This seemed to work inconsistently. I found myself having to save changes to pages twice in order to get the desired effect. Support was also less than spectacular and the peer-to-peer user community was virtually non-existent.
Bottom line. I would recommend this product, but only for organizations with a mature community and a mature IT staff to support it.