I've never been a secret shopper, but I have been scoped by one. Working retail and waiting tables has made me not care for secret shoppers too much. It's not that I didn't do my job well because I did, I just don't care to be evaluated without knowing. A lot of decent servers got fired because they forgot to pick up a plate or didn't make it back within the one minute after delivering the food to check back. Whenever we heard that a secret shopper had been there or may come a dark cloud settle over the wait staff and we waited for the hatchet to drop. Luckily, I got a decent report but, many met their fate.
However, it can be interesting because you shop for free. Yet, I wonder if it is really worth it? I will be curious to see other people's responses.