Many folks find it is NOT worthwhile. There are many mystery shoppers who only do a few jobs before they throw in the towel, most have thrown in the towel by 6 months. But there are a handful of us for whom this "clicks" and is a nice source of additional income. Very very few folks try to do this as a full time income because if you want to work that hard you might as well take a full time job that has benefits.
My guess is that at "$5-7" after going through training you are looking at CORI. While I am in no way justifying the pittance they pay for jobs, I do believe they have value because they do offer training that gives you some idea of the expectations for their jobs which can also be generalized to work with other companies. For that part of the experience and for the fact that they have jobs available in just about every corner of the country, they are worthwhile. If you ignore the pay aspect, a CORI job will give you a sense very quickly of whether you are comfortable doing mystery shops.
Recommendations to new shoppers to look at Shop 'N Check or CORI or Gapbuster or Service Intelligence or Trendsource is not being mean but rather suggesting that shoppers check companies that are likely to have shops in their area and likely to have shops that offer some reasonable training and instructions. These are also reasonable places to "get your feet wet" because once you find the companies that are smaller with fewer jobs but clients in your area and better pay, you are less likely to be looking for jobs at the "big" companies except as "fillers" or when they get bonused. There is also the aspect that if you screw up on a $4 job with these guys it is no great loss. Similarly, if that screw up gets you terminated as a shopper, that too is no great loss. If you screw up on a job where you laid out $60 expecting it to be reimbursed and they don't accept the job or reimburse you, that is a problem. Similarly, if you screw up with a small company that offers $20-30 jobs and they terminate you as a shopper, that is a problem.