I've done several restaurant type shops for them in the past few years, one every few months as their job board for my area is slim. The report form is pretty easy to fill out, short narrative evaluation if I remember correctly.
Yes, they are slow to pay like so many other mystery shop companies. I think the slow payers wait to get paid by the client before paying out the shopper, but I believe the timeline is stated up front in their FAQ's.
They are easy to communicate with by email. Many of these companies are not staffed on weekends, so response may not be immediate. Some will offer a weekend emergency voicemail through their switchboard.