Large interactive displays are increasingly used in public spaces. Yet, it is still a challenge to understand how people behave when faced with such displays in their real life, not only when they are used for entertainment (or advertising), but also when they mediate more purposeful tasks. Do people feel shy? Are they willing to interact? Are they satisfied with the services offered and will they come back to use them again? To answer such questions, these systems have to be evaluated in the field so as to understand their actual impact on users in the real life. This paper first introduces an initial evaluation framework, aimed at highlighting some of the variables involved in understanding the impact of large display installations. Then, it applies the framework to analyze users' behavior and their experience with a large display installed at an international conference. Results highlighted that people showed greater interest in those services that, despite a lower appeal, supported them in carrying out useful tasks.