We finally upgraded to a newer version of Exchange and OSX added in support for newer Exchange servers in some previous version, which finally obviated the need to use this, but back then it was a lifesaver.
I have used Exchange and didn't see any advantage at all. Perhaps I missed something.
With lync integration properly setup it's really nice as well. Though the mac client doesn't work as cleanly as it does on windows.
In my experience they are big, and tend to lean heavily on php, which isn't much of a plus in my book (mostly because of php's traditional deployment model as a web server module, and issues with session-files and db connections being shared across different scripts -- things that aren't intrinsic to php, but common enough in large project that aim to be able to install and run easily "anywhere").
I have yet to find something light weight that I prefer using, but if you really want "an alternative to Exchange", you might want to look at Kolab: https://en.wikipedia.org/wiki/Kolab ,SoGo: http://sogo.nu/ (iRedMail bundles some SoGo components) or Citadel: http://www.citadel.org.
I find that a lot of smaller businesses don't need everything Exchange does, and larger ones should just consider Exchange. It's simply a pragmatic choice, though if someone has an Exchange alternative and makes if FAR easier to setup, I'm all for it. Contacts + Calendar + Mail + Team orchestration is integrated with Exchange+Outlook better than anything else I've tried.
Thanks for all the replies. I will check them out.