We build on previous work [12], [14] on the development of a computer controlled wheelchair equipped with a suite of sensors and a novel interface for human-robot interaction. In this paper, we present experimental results and usability studies for the wheelchair. The architecture for human-robot interaction is hierarchical, with the lowest level corresponding to trajectory control, the intermediate level being behavioral and the highest level involving the composition of behaviors and navigation. Our experimental results illustrate the benefits of a shared-control paradigm where the human operator selects the appropriate hehavior(s) or goals while the software is responsible for executing behaviors and generating safe trajectories. Experiments with human users highlight advantages of augmentation in wheelchairs.