Although adaptivity, the ability to adapt, is an important property of complex computing systems, so far little thought has been given to its evaluation. In this paper we propose a framework and methodology for the definition of benefit-based adaptivity metrics. The metrics thus defined allow an informed choice between systems based on their adaptivity to be made. We demonstrate application of the framework in a case study of restart strategies for Web Services Reliable Messaging. Additionally, we provide a broad survey of related approaches that may be used in the study of adaptivity (comprising, among others, robustness, performability, and control analysis), and evaluate their respective merits in relation to the proposed adaptivity metric.