It really depends on the shop... I'll put it this way, the best way to maximize mystery shopping is taking advantage of multiple streams of revenue.
For me, if it's a decent reimbursement, but the report's long, sometimes I'm okay working for that.
If it's a low reimbursement, but the report's short, again, sometimes I'm okay working with that.
On top of that, I prefer to burn and churn my rewards I accrue across the board such as through my credit cards, as I often receive a higher redemption rate for them (1:1 for cash, but sometimes it's 1.2:1, 1.15:1, etc. for redeeming for gift cards). That alone adds up over time and sometimes maximizes the returns, depending on the shop. For example, if there's a high reimbursement steakhouse with a $200 reimbursement limit with no fee, but I'm able to great gift cards for it for a "15% discount," then I can easily make $30 on the back end wedge.
Same goes for hotels, especially hotel shops that reimburse vs. charge backs on your card. Some of the best combinations I had was when I redeemed a $60 credit for a hotel chain due to AMEX's offer, then stacked on an additional 6% back and utilize my $300 hotel credit to subsidize my card's annual fee... Plus pocketing the points and getting steps closer to lifetime status. Even though the shop had little or no fee, the back end incentives make shops like that worth it.
AND THEN, sometimes I travel for my full-time job and am given per diem. Sometimes if there's enough time, there's little to no hassle to put in extra hours on the side for the report, I'm able to double dip on reimbursements from the MSC, and then again with my company for food and/or lodging and/or transportation per diem.
Shopping the Greater Denver Area, Colorado Springs and in-between in Colorado. 33 year old male and willing to travel!