I've been mystery shopping on and off for a few years now. I really like it since I always have the freedom to refuse an assignment if I want to.
It pays to sign up with as many companies as you can (keep good records of your ID and passwords, many mystery shopping companies have similar names).
When a MS company has a big client, then the same assignments are often available over and over again. Once you do an assignment once or twice, it gets very easy after that.
I've done lots of restaurant mystery shops. The best one I had was at a fancy restaurant in a trendy part of town--it included drinks at the bar, appetizers, entree, dessert, everything. We got a $100 dinner for free, but the report took 2 hours to complete (and I type 90 WPM). It was still very worth it! Unfortunately, those kind of assignments are few and far between.
I also used to do the oil change mystery shops, but they're not doing them in my area this year
Retail mystery shops often don't pay very much. However, if it's in an area I would be going anyway I'll do it from time to time. Sometimes you get to buy (and keep) merchandise and get a small shopper fee on top of that.
All in all, I like it and would recommend trying it. I'm intending to start doing it more myself after our Sept. trip, otherwise we'll never get back to Disney and that's just too depressing!