Ambient Assisted Living (AAL) aims to improve the people's quality of life, especially vulnerable groups, with the support of information technologies, increasing the autonomy, self-confidence and mobility of the users in their daily activities. Hence, it is essential to know the end-user's opinion about the perceived quality when using an AAL system in terms of the degree of satisfaction of their real needs and achieved objectives. However, since AAL is a relatively new domain, existing quality models are limited, with an incomplete set of attributes and metrics. Therefore, this research proposes a quality in use model for AAL systems focused on the followings characteristics: effectiveness, efficiency, satisfaction, freedom from risk, and context coverage; and their corresponding metrics adjusted to the AAL domain. Finally, the proposed model has been applied as a guideline during the evaluation of an intelligent pillbox. The results of this application have proven that this model is a valuable tool for evaluating the results and effects of using an AAL system from the perspectives of either a primary user (e.g., elderly people, people with special needs, patients) or a secondary user (e.g., family, caregivers). Consequently, it identifies essential aspects that contribute to improving the quality of those systems.