I do not use surveys. I have a few long term clients that are not good at self motivating. But most of my work is aimed at educating clients to be self sufficient in their exercise programs. As I have a high rate of clients returning for progrssion programs, I do not feel that a survey would be very useful. Though I do tend to contact clients regularly and discuss how they are doing and if they have questions etc.
If I was having issues with client retention or dissatisfaction, I would probably use a survey. Or at least initiate some type of program to resolve the issues keeping me from attractng and keeping clients.