Does anyone have any experience as a secret/mystery shopper?

I was just curious what this industry is like. It seems like a decent way to earn a few extra bucks but I'm not really looking to commit too much time to it either.

Can I just pick up jobs as I want to do them or will they boot me out if I don't complete enough surveys?

If it's for a restaurant or something, would I be able to bring my family along?