Hey Folks - We need your help. We were hoping, we could gain from your experience with implementing server virtualization. When should we be using Red Hat Virtualization, Citrix Virtualization, Xen Virtualization, Microsoft Virtualization, Oracle Virtualization and VMWare. I will be very interested in learning from your first hand experience. I will also like to know, under what situations is one virtualization preferred over the other. - Thanks for your help

I use VMWARE ESX 3.5 and it is a great project.
We are only in our first year with major plans in the next two years but it works great and is very stable.
The cost on the number of physical servers you need to purchase, the cost of multiple physical servers being an asset, the cost of cooling and powering multiple servers (utility, UPS, generator power) is greatly reduced.

Of course, you do buy more powerful servers and VMWARE licensing per processor is expensive but if you have many servers, you will start to see an ROI sooner than you would think.
Most servers are not fully utilizied so running virtual instances (servers) on a physical machine allows for better hardware utilization.
About the only thing you need to keep an eye on is disk utilization so get 15,000 RPM SAS drives.
Also, a NIC with 4 ports or multiple NIC's is a good idea to keep network load at a good balance between the virtual instances.

It is best to have multiple of the same servers and use a SAN. That way, you can take one physical machine offline for whatever reason and let the other machine handle the workload temporarly without taking down your virtual instances.

I don't know if I answered any of your questions; I am just typing.