I spent some time today upgrading a Zimbra (OS edition) installation from 4.5 to 5.0. I run it an install locally and a client also runs it on-site for their own mail.
The client in question wanted to be able to share mail folders between users, something that version 4.5 never did on the "free" version. However, 5.0 does.
For me this means I no longer have to setup a thousand accounts and filters in my MUA to redirect a million billion daily cron emails, but can simply share the folders on the system notification account from within Zimbra and have them available on whatever imap client I care to use with just one single account.
Something else I just noticed is the ability to subscribe to RSS feeds as mail folders - again allowing the use of any MUA to read feeds.
If the hot backups that Colin Charles mentions go into the OSS edition, I'll be truly happy. For now though I'll need to stick with using LVM snapshots for backup and upgrade insurance purposes.