If I’m a customer, I have to seriously revisit the Microsoft option. It?s 59% cheaper, does way more across the entire application stack, and the focus is on the business application in the private cloud, not on the irrelevant (yeah I said it) hypervisor layer that can probably fit on a tiny disk.
vTax obviously overblogged/hyped in the last week and to be fair vSphere 5 is a strong if storage centric upgrade to the industry-standard virtualization platform. That said, vTax is a legitimate red flag for RAM heavy virtualization deployments (ie most). As of June 2011, I would recommend vSphere 4.1 for 95% of use cases. It was [and at time of writing still is] best of breed. The vSphere 5 announcement changes that recommendation. For Windows centric enterprises, it is prudent to evaluate VMM 2012 (in beta, expect it this year) and Hyper-V as an alternative to moving to vSphere 5. The platform (with VMM2012 release) is both functionally and economically competitive enough to warrant a POC.