It is the crème de la crème of candy stores. So.much.merchandise. Anything and everything you ever wanted with an M&M logo on it is here. Towels, bowls, plates, backpacks, coffee cups/mugs/tumblers, t-shirts, stuffed toys, lunch boxes, you name it. On the 3rd floor is a cute little "4D" movie with lighting and wind effects, and live action - all free.
It's a must-do if visiting. And all of the screaming kids is a lot more tolerable if you drinking from one of those yard-long souvenir margarita containers (sold next door at the La Salsa Margarita Bar and yes you can bring it in the store).