Hardware and software situation :
- Exchange 2003 vSphere VM running Windows 2003 R2
- ZCS 7.0 on a CentOS 5.5 x64 installation - 3 cores and 4 gigs of ram dedicated to it
- IBM x3650 with 2 dualcore Xeon 3Ghz (so 2x 2 cores) and 12 gigs of ram (both for the old exchange and new ZCS)

Don't think i need to say running Exchange 2003 these days is a bit hardcore, but the company i work for went private a few years back - so up to now we've been running on extremely cheap EDU licenses. Needless to say we needed a valid alternative to migrate to and after a few months of 'testing the waters' (mainly trying to get Scalix to run properly) - Zimbra provided a perfect alternative for my users.

Mailbox and calendar migration was flawless and despite running into a few small configuration annoyances getting everything up and running with full mailbox migration only took a few days. We decided to opt out of having people use Outlook - so it remains to be seen if people get used to the Zimbra Desktop and Webclient, but i have high hopes