This paper evaluates a design-driven, tool-based approach, named DiaSuite, dedicated to developing IoT applications. Specifically, we measure the usability and the learning cost of DiaSuite as a first step to assess the potential for transferring this technology to the industrial practice of this domain. We assess the learning cost to use DiaSuite by involving four professional programmers in a controlled experiment in software engineering. This experiment brings preliminary evidence that the DiaSuite technology can be used effectively by professional developers after only half a day of training. We then present qualitative data about the usage and usability of DiaSuite, collected from developers, and using questionnaires and interviews. Finally, we discuss lessons learned from this work.