Yes, I have. Though I haven't done it in a while, it was a nice bit of part time income. I located some secret shopper sites on the internet, and signed up with those for whom it was free to register. I started out slowly, doing one shop every couple of weeks, and, as I did more, I received more notifications.
I would do shops on my day off, taking as many as I could, two, maybe three, in an afternoon, sometimes making it a day with my wife and granddaughter. I did shops at fast food restaurants, malls, hardware stores, gas stations, etc. When I was done "shopping," I filled out a report online, and sent it. The report gets graded, and then they send you money. The money added up, too, though it didn't make me rich, it was a nice supplement.
Because of time restraints, I haven't been able to do any for a while. If you search, there should be an index of companies that need people to mystery shop. I went with a few who didn't require me sending them any money, and they offered me shops almost immediately. You may accept as many, or as few, assignments as you want. The important thing is to do the shop once you give a commitment stating that you'll complete it, thus don't sign up for a shop, and then not do it - that's the main thing.
The next most important thing, and last step, is to follow the directions to the letter. For as little time and money that I spent doing it, I received between 100 and 250 every month. Time constraints has kept me from doing secret shops for almost a couple of years. I do look forward to doing more in the future - just not sure when exactly.