I have had the opportunity to perform a few VMware Capacity Planner assessments over the years and I have been, more the most part, pretty happy with the process and the results of the reports. The assessment is really pretty straight forward. We had physical servers to the project, making sure we have proper permissions to perform all the tasks and then let the process run over an extended period of time. For the most part, this way of sampling over an extended time frame will give you a very good idea what can be virtualized and the number of hosts that will be needed.