Anyone in the customer service or marketing industry should become a mystery shopper at some point in their career.
Companies such as gyms, restaurants and banks will hire a mystery shopping company to provide shoppers who go undercover to evaluate their service.
As a mystery shopper you're provided with a set of criteria that the company is looking for you to evaluate and provide detailed feedback (with detailed being the operative word) about the service you receive. You at NO time reveal yourself as a mystery shopper. It does require an excellent memory to keep track not only of what you're expected to evaluate, but the nuances of the service you receive to write up on your report. The mystery shopping company reimburses you for your time, and/or an allotted amount for your expenses. I've done mystery shops with restaurants and been reimbursed for a complete dinner for 2. Every shop and every company is different. If you don't like the reimbursement, then just pass up the shop.
Two good companies to start with are A-Closer-Look.com and Kern Scheduling. or check out this directory of mystery shopping companies. Never pay for a list of mystery shopping companies, and never pay to be a mystery shopper.
So what can you learn from this experience? You will get a different perspective on the criteria (conscious and unconscious) that people use to evaluate your company, and will look at the service you're providing through fresh eyes. In today's competitive environment where companies are vying for every dollar, any way to provide a better service relative to your competition is a good thing. It also makes for an interesting topic to discuss in your next job interview.