With the beta word processing and spreadsheet functionality in Zimbra 6, ZCS now has the features to go head-to-head with Gmail and Google Docs.
One "feature" Google has that Zimbra doesn't however is the right to use everything you put in Gmail and Google Docs, any way they want, including republishing your content publicly.
I've just done a blog post about this, with links to the Google Terms of Service (see Section 11 for the gory details).
You can read the post here.
Hope that helps,