Bacere: any advert that asks you to pay to register IS usually dodgy. I don't think there is less work now, the work has changed though. There is more video and audio shopping now. I used to do this when you did the visit and send the footage in. Now more companies are wanting a report as well which I think is rather naughty so I'm not doing video much now.
I know what you mean about the receipts and surveys asking for feedback. But remember as a shopper they will usually check your report against the CCTV and many times the member of staff will received feedback (and sometimes bonuses) based on your reporting, so it's more detailed than surveys.
Many times I have been somewhere and had such crap service that I have thought "I wish I was MSing at the moment". I've also MSed places and heard one member of staff say to the other that they've had a call from such and such a branch and the mystery shopper is coming today and they then give a description that is so not me. I've also had someone say "Good job you're not mystery shopping me".
Some interesting ones: booking a payment plan at a funeral directors, having eye tests, going ten pin bowling, cinema visits, tes driving a sports car.
For varied work, it's a great job!