Metrics have long been used to measure and evaluate software products and processes. Many metrics have been developed that have lead to different degrees of success. Software architecture is a discipline in which few metrics have been applied, a surprising fact given the critical role of software architecture in software development. Software product line architectures represent one area of software architecture in which we believe metrics can be of especially great use. The critical importance of the structure defined by a product line architecture requires that its properties be meaningfully assessed and that informed architectural decisions be made to guide its evolution. To begin addressing this issue, we have developed a class of closely related metrics that specifically target product line architectures. The metrics are based on the concept of service utilization and explicitly take into account the context in which individual architectural elements are placed. In this paper, we define the metrics, illustrate their use, and evaluate their strengths and weaknesses through their application on three example product line architectures.