I've been doing mystery shops on and off for about five years now. I mostly do fast casual restaurants, just one or two a week. I swore them all off a few months ago, because they weren't really worth my time, but I've come back to do the ones I enjoy (meaning, the food is good, the money is decent, and the report is short).
I work with Dynamic Advantage, Marketforce, A Closer Look, and Sentry, currently. I'm checking out a few others, but it's hard to find the companies that do the shops you want. Then when you find a good gig, they restrict you to doing it every 3-6 months. Ugh!
I live in Southern California. If anybody has some recommendations that I haven't already signed up with, I will use you as a recommendation, and vice versa! Let me know what you like about the company or the shops. I'd specifically like to do more coffee shops and pizza, and other fast casual or quick and easy retail.