Think of them as similar sporting or gaming card sets. You usually get two pins per box and sets are typically a 20 pin set. As earlier stated, there are various kinds of mystery box sets. Some are open edition, limited release, and, recently, nearly impossible. 🙂
They tend to be really hot for about eight months or so, until everyone completes their sets. The popular ones, like the current Pixar Myster Box Sets, are extremely good traders. Unfortunately, they are also very good pins for flipping and as such are extremely hard to get.
I like the regular 20 pin mystery sets, as they are fun and fairly easy to complete. They also tend to garner you automatic traders, since it is highly probabable that you will get duplicates. If you are starting out and don't want to deal with LE pins just yet, I recommend them as easy traders while the set is newer and in somewhat high demand.