Physical visualizations have been around for several decades and remained mostly unnoticed. They recently became popular in the form of data sculptures, due to a proliferation of data-driven artifacts produced by the art and design communities, and to a wider availability of rapid prototyping facilities such as fab labs. It has been recently suggested that such physical data representations are suitable for demonstrative, artistic or communicative purposes. But can physical visualizations also help carry out actual information visualization tasks? We describe the design of the first user study whose goals are to assess the efficiency of physical visualizations compared to on-screen visualizations - with a focus on the challenges posed by 3D visualizations - and to better understand how people use physical data representations to answer visual questions.