Graphene nanoribbon FETs (GNRFETs) are promising devices for beyond-CMOS nanoelectronics due to their excellent carrier-transport properties and potential for large-scale processing and fabrication. This paper combines atomistic quantum-transport modeling with circuit simulation to perform technology exploration for GNRFET circuits. Results indicate that GNRFETs offer significant gains over scaled CMOS at the 22-, 32-, and 45-nm nodes, with over 26-144× improvement in the energy-delay product at comparable operating points. A quantitative study of the effects of variations and defects on the performance and reliability of GNRFET circuits is also presented. Simulation results indicate that whereas GNRFET circuits promise higher performance, lower energy consumption, and comparable reliability at similar operating points to scaled CMOS circuits, they are more susceptible to variations and defects. These results motivate significant engineering, modeling, and simulation challenges facing the device and computer-aided design (CAD) communities involved in graphene electronics research.