The usability concept refers to aspects related to the use of products that are closely linked to the user's degree of satisfaction. Our goal is to present a functional evaluation methodology for assessing the usability of sophisticated technical aids, such as a portable robot for helping disabled patients with severe spinal cord injuries. The specific manipulator used for this task is ASIBOT, a personal assistance robot totally developed by RoboticsLab at the University Carlos III of Madrid. Our purpose is also to improve some aspects of the manipulator according to the user's perception. For our case study, a population of six patients with spinal cord injury is considered. These patients have been suffering spinal cord injuries for a period of time longer than 1 year before the tests are carried out. The methodology followed for the information gathering is based on the Quebec User Evaluation of Satisfaction with assistive Technology (QUEST). Different daily functions, such as drinking, brushing one's teeth and washing one's face, are considered to assess the user's perception when using ASIBOT as a technical aid. The human factor in this procedure is the main base to establish the specific needs and tools to make the end product more suitable and usable.