THEMIS-AR is an augmented reality (AR) app designed for mobile devices that was developed to assist first responders on disaster relief operations, which a very demanding context for users, regarding both the physical and the cognitive and emotional demands. In fact, the specificity of the usage environment imposes special care in addressing the interaction of the users with the system, regarding both the mobile devices technical characteristics and the workload. The paper provides a brief description of the THEMIS-AR architecture and features, focusing on the assessment of a concept demonstrator developed based on the UCD cycle (i.e., context of use characterization, requirements definition, solution implementation, and testing and validation). The usability testing setup and script adopted while performing field tests are described and the results obtained, namely regarding the application of the User Experience Questionnaire to benchmark the app evaluation in comparison with a set of previous evaluations made with this methodology. These results were compared with the user perceptions collected using a final questionnaire namely based on the System Usability Scale. The main findings resulting from this initial set of users are summarized and presented the way ahead for this component of the THEMIS project.