I've found if I make my own patties and work in some cumin into the meat plus a little bit of onion, they come out really juicy and flavorful. The cumin used all by itself does not impart a 'mexican' flavor, more of a sage kinda flavor. I don't know what it is about it that makes the burgers juicier, but it does. I've made burger with onion in them before, but adding the cumin is what makes the difference.