In this paper we introduce a subjective metric for evaluating the performance of spoken dialog systems, Caller Experience (CE). CE is a useful metric for tracking the overall performance of a system in deployment, as well as for isolating individual problematic calls in which the system under-performs. The proposed CE metric differs from most performance evaluation metrics proposed in the past in that it is a) a subjective, qualitative rating of the call, and b) provided by expert, external listeners, not the callers themselves. The results of an experiment in which a set of human experts listened to the same calls three times are presented. The fact that these results show a high level of agreement among different listeners, despite the subjective nature of the task, demonstrates the validity of using CE as a standard metric. Finally, an automated rating system using objective measures is shown to perform at the same high level as the humans. This is an important advance, since it provides a way to reduce the human labor costs associated with producing a reliable CE.