We just successfully set up 15 client users to zarafa recently last weekend. Let me tell you it wasn't easy they had HUGE mailboxes one user had 17GB on his own while their public folder was 21GB.
We just got it all moved over by Wednesday using imapsync this week now and I have to say I'm quite impressed with the performance of having such large mailboxes and still be quick and efficient for the end user. Everyone is quite happy with the shared mailbox features for the public folder and how quick it is to access them.
Also the backup incrementals are a very nice touch too and it doesn't take that long to backup the 21GB which that in itself is impressive. All features they want i.e. z-push imaps are all set up and they seem very happy. I hope that in the future we can keep on getting more clients to move to this amazing product as it's definitely the next big thing. After seeing this in motion I can definately say that Exchanges day's are numbered!