I was going to post the Volition link, but I see that someone already has

It's a GREAT resource and very legit.
From my personal experience ... yes, MSing can be profitable ... but you have to be selective in your shops for it to be worthwhile.
No way am I going to take a revealed FastFood reward shop that requires a 8 page on-line report, several to-the-second timings, measuring the liquid in my soft drink, and revealing my self to either award prizes or give a 'sorry you missed out, review your suggestive selling materials and try again you never know when another shopper will drop by' kind of certificate ... all for reimbursement for a fastfood combo meal and a single digit fee! No way ...

... and I've reached the point of passing on most cell phone shops, fastfood shops, home improvement store shops, office supply store shops and a lot of others that while possibly worthwhile if you have the time and they fit into your daily schedule and aren't too far from home/work ... but they just don't pay enough for me to try to work them into my busy summer schedule or justify a trip across town or to a neighboring town with gas prices as high as they are now.
However, I *WILL* happily take shops that provide me with 6 Orlando area theme park tickets

, or Atlanta area professional sporting events, or $100-$150 reimbursment for a fine dining meal (for me & hubby), or even $35 for a local restaurant meal with the family, or just under $20 for grocery store reimbursement & fee for a super simple shop at a store that I'd be going to anyway and that with coupons I ended up with about $54 worth of groceries!
There're all sorts of mystery shops out there and all sorts of different reimbursement/fee schedules. Sometimes starting out it's worthwhile to take some of the lower paying ones just to get your foot in the door with a company ... but what is really 'worth it' for one person may differ from what is worth it for another.
One year I did several local department store shops just in time to buy my kid's back to school clothes. The shops required observing/reporting on 8-10 different associates, but you didn't have to get a lot of info just their names and a little other info ... and I found it worth it for the fee. By shopping sales and clearance, I think I ended up with about $400 worth of clothes (if regular price) over the course of 5 shops which paid $20 each. I may have spent a tiny bit more than the $20 on some of the shops, but I didn't spend a lot more. It took a little more time than if I'd gone to the same store to shop without doing the MS, but it wasn't awful. It was worth it to me because of hubby's job change which had made our budget pretty tight.
However, I've passed on a the shops since then because money isn't as tight and I just dread trying to make contact with 8-10 associates in a store which seems to barely ever be even close to fully staffed. If I needed the money, I'd do them ... but they just aren't that appealing to me and seem like more trouble than they're worth. But I know the year I did them, I was happy to do them. Does that make any sense?