This is not a big income proposition. Fees per job average $10-15 and require you to spend $1-5 on average to be reimbursed. The average job takes two hours for the first one you do--between taking certifying tests, studying the instructions, doing the job on site and reporting it on your computer with uploads of receipts, photos and whatever is required. Subsequent performance of the same or similar jobs will likely cut in half the time of the first job you did because not always do you recertify and often the instructions do not change from your previous shop. Reporting times also become quicker after you have done the report several times.
Making a career of mystery shopping is not feasible in most markets as there are not enough evaluations to be done.
To do this work you need to spend a fair amount of time looking at job boards to find work to apply for. You need to work within fairly limited time windows to get the work done. You need to supply all your own equipment and supplies as well as cash up front to make required purchases. You need to provide your own transportation, internet access and such.
It can work well as supplemental income depending on job availability in your market and your observation and reporting skills. Job availability is a key issue during this time of economic downturn as many clients have curtailed or eliminated their evaluations.