MINWii is a music therapy game for the renarcissization of demented patients. It lets players improvise or play songs of their choice by pointing at a virtual keyboard with a Wiimote Pistol. We present the results of a three-month usability study we conducted with 7 institutionalized patients suffering from mild to moderately severe Alzheimer's disease at the LUSAGE Living Lab in Paris. We demonstrate that MINWii is indeed usable by AD patients despite their motor and cognitive impairments: our results, which were largely computed automatically thanks to MINWii's extensive logging capabilities, show either an instant mastery or a clear learning effect depending on patients' cognitive abilities. Moreover, patients were overall very satisfied with the game and expressed a desire to repeat the experience: MINWii fosters positive interaction with the caregivers and elicits powerful reminiscence with even the most severely impaired patients. This study justifies future research to assess the lasting effects of playing MINWii on both quality of life and cognitive impairment in demented patients.