I used Citadel a few years back. It was fairly easy to install and easy to manage, although there were definitely some quirks in their terminology which made figuring it all out a little challenging. However, the reason I stopped using it was the web interface for all of the functions (mail, calendaring, etc.). It was very strange to say the least. Just didn't feel like a normal mail server like Exchange or Zimbra. I think it was based on the old BBS model or something. I also seem to remember some odd IMAP behavior, but I can't say specifically what it was since it was about 3 years ago. I've since switched to Zimbra and am very happy. While there are some frustrating bugs (e.g. installing certs) it has proven to be very stable and easy to setup/administer.