I intend to develop a smartphone app for carers of people with dementia to help in managing functional disabilities of their care recipients and to conduct a feasibility study. In the feasibility study, I intend to assess user satisfaction.
Just develop questionnaire how satisfied they are, or how frequently they use smpart phone app. Develop questionnaire Likert scale way to make it more applicable and to consume less time. Be specific who are your target population elderly suffering from dementia or people taking care of them.
I believe Likert scale is an efficient way to find out satisfaction level. However, depending on what kind of smart phone app you are researching about, you might want to ask about specific items/statements rather than satisfaction towards the app in general. For example, for a messaging app, I would ask about items like color, text sending speed, app opening speed, etc.