I just want to share my experience. I have zimbra run as VM on a potent host with loads of RAM and unfortunately rather weak I/O-performance (driver-issues with my sas)
The first year I had it run as vmware-server-machine and it was a mess. Especially after upgrading from zimbra5 to zimbra6 system-load on the host went up to dangerous levels. Since then zimbra was getting slower and slower and the hostload bigger and bigger.
A week ago I moved the whole system from vmware-server to vbox and ... voila ... sysload down and zimbra responsive again.
Not sure if its a zimbra-issue with vmware (like some people suspected after 6.x was released) or if its just something on my machine.
Host ist Ubuntu 8.04-64 on a dell R300
Guest ist Ubuntu 8.04-64 as well.