Last year we moved from Exchange to Open Xchange and a few months later to Zimbra.
It was a lot of work but we made the right choice. Centos + Zimbra is definitively a great and reliable option.
So anyone out there still hesitating to make the switch, I say go ahead, you will save a lot of money for IMO a better product.
I had to learn the hard way because I wasnt a Linux expert (Im still not), but it is possible to make the switch if you are willing to learn. This forum , particularly Mr Hammonds and Phoenix (avec qui je suis en désacord avec certaines de ses pratiques ) helped me a lot, thanks to both you and to everyone else is helping other "noobs" in the installation forum.
Now I have 3 servers running Zimbra and everyone is happy.
Btw, the hardware I use for each server cost less than 800$ (but I have 2 NAS (raid 5) for the backups).
Pass the word everyone, Zimbra is a great software !