Because I do this full time (video mainly) and I need the money for living essentials - not just nice to have's, I need to take a logical approach to mystery shopping. Living in a big metropolis helps a lot - there's plenty of work and choice, so I can make it a full time living - as long as I am smart about it.
Starting point: the best MS job to have is one that pays a lot but takes as little time as possible (including travel and report writing time). A bonus is that it is fun to do as well and/or it is at a place where you need to go anyway. For many jobs, they don't tick all of these boxes. Hardly any do, in fact! The fun ones are low paid or involve a lot of travel and petrol (gas you you Americans) or they are right next door and boring a f**k and the report is a like writing a novel.
For those of you who love restaurant/cafe jobs or tend to do these quite often only makes sense to do them as 'incidental' visits (i.e., you were going there anyway, or at least passing that way - negating the need to factor in travel time or even the time spent in the restaurant because it was either doing it for a bill (cheque to you Americans) or doing it free of charge. That means that you are substituting some observations and report writing for the bill (or most of it, if a contribution), which makes it a positive sum game and worth doing, particularly if it is not a special occasion or you are alone.
If you were not going there anyway, making it a destination mystery shop (i.e., you are going to work - you wouldn't be going there at all or at that date/time unless it was a mystery shop either as a stand-along job or part of a route of shops), you need to consider whether the food is going to be a high enough or enjoyable enough standard for you to feel adequately rewarded for the effort. If it is just a fast food or unappetising stomach filler, think again - unless your cupboards are empty and there is no cheque coming in to buy groceries. If it is not to fill up cupboards and/to prevent hunger, and is somewhere wonderful or enjoyable it might be worth considering doing some other types of work that pays more just to pay this restaurant bill, so that you can enjoy the meal more without having bothersome timings, observations and a long sluggish report to do later (and the risk of not getting paid or reimbursed if you miss a vital observation or name tag - making it a huge cost to yourself. All that happens when you do a destination restaurant visit, unless it is somewhere really swanky and out of your usual price range for a meal out - you are just adding risk and having a less than great time in exchange for the bill, which makes it a zero sum game (if you are paid, neutralising its worth) or a negative sum game if you lose the fee/reimbursement and/or just hate the food.
Even when done well enough to be paid and reimbursed in full, restaurant visits tend to give a low fee and expect a long report, so it needs to be worthwhile to visit. I would say that most of the time they are not worth it, unless you are a regular customer at one of your mystery shop destinations - and are bagging a freebie instead of going to work to complete it as a destination shop.