In my case, not much. I've worked up a cost model that I used for estimating and then actualizing the project I'm deploying VMware ESX server for. I'm going to sanitize it and scale it out to see where the break points are in terms of cost and configurations. My guess is that there will be a break point for standardized configurations, but mixed configurations are going to be harder to justify. This could get really interesting. I'll even publish some pretty graphs and charts.
I'm getting ready to deploy VMware ESX Server in our data center. I've used VMware in the past and had great success with it, but only in test and development environments and only their GSX server. We are using ESX for an internal application that doesn't require the service level our external applications do. This should be interesting. I'd love to set up an ESX server with a few VMs on it and do some performance testing against it. Does anyone have experience or comments specific to ESX VM performance? Any input or war stories would be greatly appreciated.