Mystery Shopping?

I really can't tell you how I stumbled on this, but stumble I did and now I'm curious to see if anyone else has any experience with Mystery Shopping? I found some interesting articles on Stretcher.com, including one that describes in great detail the inner-workings of the biz, another with some handy advice from the horse's mouth, and another article of a woman who has done it for 20 year and has some handy tips. It's the kind of thing that isn't a full time job, but if you work it right, could get you some extra cash or an excuse to go out. Methinks I'll check this out when I gets back to the land of the free...