We have developed a self-consistent atomistic simulator for CNTFETs. Using the simulator, we show that a recently reported high-performance CNTFET delivers a near ballistic on-current. The off-state, however, is significantly degraded because the CNTFET operates like a non-conventional Schottky barrier transistor. Design optimization for significantly improving the off-state is explored. We have also assessed how the CNTFET compares to the state-of-the-art Si MOSFET and the potential of CNTFETs for high-frequency applications.