Unmanned vehicles (UVs) are being developed and fielded at an unprecedented rate in various environments for both civilian and military purposes. Despite the term "unmanned," control of such vehicles requires considerable inputs from those operating the UVs and others in the C2 system. To optimize such mixed initiative teams we must develop new methodologies and measurements for evaluating and understanding human-robot team performance. The Mixed Initiative Team Performance Assessment System (MITPAS) provides such new methodology. MITPAS consists of models, tools and procedures, including an OneSAF-based simulation environment, with which to measure the performance of mixed manned and unmanned teams in both training and real world operational environments. This paper describes MITPAS and the results of several initial experiments conducted to validate the measures and gain insight into the effect of robot competence on overall human-robot team performance, operator trust and operator situational awareness. Our initial results are indicative of the type of new insights into human-robot team behavior that can be gained by combining the measurement power of MITPAS with realistic simulations of tactical UV operations. Consequently, we are working to make the readily-customized MITPAS available to other researchers and developers for use with their simulations, scenarios and special measures.