I signed up for National Shopping Service a while ago but just recently completed (and was paid for!) my first shop. I found the directions to be clear and straightforward and the shop form was fairly easy to enter. In my area they have quick casual restaurants and retail establishments primarily. The site is easy to navigate. I'll definitely sign up for some more shops with National Shopping Service!
As always, it's completely FREE to sign up. You won't really receive emails when shops are open so you should plan to check the site a few times a week.
To learn more about mystery shopping, please read my Mystery Shopping 101 post. For a list of other mystery shopping companies I have discussed click here.