Yesterday I set up my gmail account as an external IMAP account in Zimbra. All seems to work very well. However one thing I noticed may need some attention. I did a global search using is:anywhere <search term> To my surprise it searched inside my external IMAP account (even down to the attachments)!
This is a truly great feature for data aggregation - but I wonder how long the indexing would take if I had used the whole of my 5GB gmail allocation. It would may be useful to have an option to disable indexing on external accounts, or a default limit to the number of items read. As it was the initial sync of the accounts took a longish time compared to a typical IMAP sync of just the mail headers of the external account, and I suspect this might be why.