my GF did it for a year or so.
my advice: don't do it.
i believe you get reimbursed. you generally have an assignment to buy a particular item at the shop of interest. you have ot send a copy of receipt to get refunded + the $10 payment on top of that. for many assignments, this $10 represents ~ 4hours of work including going to the store and writing up the report. the people who review your reports are major asshats. considering that the majority of mystery shoppers aren't necessarily college grads, their requirements for properly written reports are ludicrous. (I wouldn't consider the reports that difficult, just that the people reviewing them and hence in charge of your rating--which is crucial to your ability to gain "lucrative" shop opportunities--to be king asshats. they realize they have this power, and they flaunt it) there are better ways to earn pocket change: Ebay, for example.
the mystery shopping community seems to be highly competitive, sometimes desperate for these positions. register and read the forums to get a good idea of what it's like. there seems to be a housewife pack mentality going on.
I would say that it does help you become more observant, and help with memory. My GF did it when she first came into the country, and to help her with her conversational English (her 5th language). If the reviewers suspect in any way that you blew your cover during your assignment, they won't pay you.