I'm using Server 2003 64 bit has a host. It's been in use for almost a year and runs flawlessly with many different guest OS's including Server 2003, Server 2008, XP and Win2000.
Only Windows 7 gives me any trouble.
I have 16GB of Ram on the Host, and have allocated 2GB to the guest. I'm not sure where to look in the vmware logs to find anything useful. The Guest just locks up and requires a reboot. After the reboot the Windows 7 guest runs fine for a while and then crashes. I'm ususually using Visual Studio 2010 when it happens.
Of course, VS2010 is a beta, but I would think that a new, shiny OS like Windows 7 would be able to let bad apps fail gracefully without bringing the whole OS down, right?