After working for several mystery shopping companies, Grass Roots are, in my eyes, by far the best.
If you are thinking about performing mystery shopping work, be aware that it is usually issued on an adhoc basis - you'll get bursts of it, then nothing for ages. I usually perform a few assignments a month for Grass Roots and these are well paid for what you have to do. When there are jobs in your area, Grass Roots will email you giving you a brief description of what the assignment entails, where and when it is to be performed and how much you will be paid to do it. (as well as how much you will be reimbursed if you make any purchases.) It is then up to you whether you decide to accept it or not. There is a help-line if you are unsure of any aspect of an assignment and the staff are always friendly and polite. I have always been paid promptly too.
Overall, I would reccomend Grass Roots to anyone who is thinking about applying to be a mystery shopper, but be aware that the work can be few and far between.