I haven't seen many shops from Guest Check in quite some time in my area. I just saw some from a job board, but I would not do them. Their pay is rather meager for the time spent and the narrative. More importantly, they will only question your receipts if it benefits them. (I once submitted an obviously wrong receipt, but because it was much lower than what I submitted in my itemization, they paid me the lower amount and basically said - "Too bad." And this was after I had already done about ten shops for them and never had a problem.
Well, they saved their $10 but they never got me to do another shop for them. Penny wise, pound foolish imo.