Having now used Yahoo's open source Zimbra email server to host my email for some time, I am finally getting around to writing some words about it.
As a no-cost solution, running on comodity hardware and an open source software stack supporting it, I would chose Zimbra over any of the hosted solutions I have come across for the 1-50 user business (or in my case family). The rich webmail client has replaced desktop email clients for me, and having access to the same mail client, with the same customisations and settings from any machine with net access is a great boost. More recently the full desktop client, which allows offline access to messages, is easily as feature rich and useable as solutions from Microsoft or IBM that I have had the opportunity to use in anger, as well as the various open source solutions out there - can anyone say "evolution --force-shutdown".
There are features that I miss - GPG and S/MIME signing and encrypting my emails, ctrl-click drag and drop to copy messages - but progress is good and you can always connect your favourite desktop email client.
One more promising bonus is the ZimbraME Beta product - this installs on your Mobile Phone and provides you access from anywhere to your mail, a great addon. I have the generic version running on my Three Skype S2 phone (although scrolling within messages doesn't work) and for a beta its great to check which parking space I have been allocated when I turn up at work first thing like a zombie!
With version 6 just around the corner, lets hope my high expectations continue to be met!