i did a few years ago and it was really handy. However, I now find that you have to pass a test to get in and the last time I tried (a few years ago) I failed!
It is a really good thing to do if you can get in, though, as you get all sorts of freebies. I used it as a way to go out to eat for free. It was good for me too as I have opinions about everything so with mystery shopping I could tell them what I thought as well as getting paid!