They do them every few months or so. I've bought one in the past when I was just starting to get into BM so seemed a good risk to take as I only had their foundation. In my lucky bag I got a mascara, blush/all over face powder, eyeshadow, eyeliner and I can't remember what else. It's good value if you use what you get, but of course that's the gamble - you don't know what you're gonna get!
You always get full sizes and they don't put things in that are skintone specific like foundation or concealer. You can pretty much guess that there'll be a mascara in there and some kinda blush and eyeshadow.