I really like SQM. Their scheduler, Betty in my area, is amazing. Easy communication, whether by email or phone, shops with clear instructions, simple reports and they always pay on time. Also, they have a great variety of shops available ranging from fast food, dine-in restaurants, flights, hotels, rail service, phone and email shops, and many more.
The only issue I have with them is the need to "make" an invoice at the end of the month. One must "send" (it's all done internally on their site but the onus is on the shopper to click at the right place to have their invoice validated) the invoice before the 5th of the month if one wants to be paid that month. If one forgets, as I often do, one will only get paid the following month.
They get a nice big thumbs up from me