This is one of my favorite mystery shopping companies. They have very easy online forms and the shops typically are not too in depth. They pay on time every time and their fees are par for the course - not too high, not too low.
Shop N Chek has a large variety of clients, but not a ton of shops (at least in my area) because the shops they do seem to be only one time a month.
Shop N Chek also offers some merchandising opportunities, which have also been fairly painless. Check them out!