We apply quantum error mitigation (QEM) techniques to a variety of benchmark problems and quantum computers to evaluate the performance of QEM in practice. To do so, we define an empirically motivated, resource-normalized metric of the improvement of error mitigation, which we call the improvement factor, and calculate this metric for each experiment we perform. The experiments we perform consist of zero-noise extrapolation and probabilistic error cancellation applied to two benchmark problems run on IBM, IonQ, and Rigetti quantum computers, as well as noisy quantum computer simulators. Our results show that error mitigation is, on average, more beneficial than no error mitigation—even when normalized by the additional resources used—but also emphasize that the performance of QEM depends on the underlying computer.