It's nice to know what the good companies are, it would also be helpful to know what geographical area some of them cover too. A great company in Texas doesn't do me any good in Illinois.
In northern Illinois I have done shops for 13 different companies so far. They have all paid within the time frames they gave me but not all of the time frames are as quick as some would like. My favorites so far are Certified Field Associates, Speedmark, Bestmark and Second to None. For Certified all I do is the Trailer Checks, it is easy money to just write down what the trailers are. I work afternoons and weekends in my full time job so I never do the audience reaction and patron counting checks so don't know much about them. Certified is also good if you like doing Merchandising shops.
I did one for NSite that was only a reimbursement, but it was to a place I had wanted to check out and my friend and I had a really good time on their dime,but they only pay 30-45 days after the shop and the only one I have done was paid on the 44th day. Other than that one, I haven't found too many in my general area.
Other companies that have shops in northern Illinois are Intellishop, aTH Power, Shop n Check, Spies in Disguise, Beyond Hello, BARE International, Corporate and Service Performance Group.
Most of the Corporate shops don't pay well enough to make it worth the trouble, but they are good starting place for new shoppers to get the experience.
My advice to new shoppers is to only take the shops that just reimburse expenses if it is someplace you like to go to anyway. Most of them are restaurants and you are allowed to take a guest so it is a good way to go out to eat with friends while on a budget. Also, if you don't have a flatbed scanner, it would be worth your while to get one, they are not really expensive and most of the companies want you to upload your receipts.