Back when I worked in restaurants, I remember some of the best fries I had coming into the place frozen. These were not the skinny McDonald's type, but much thicker. The cooks dumped them straight into the fryers and they came out beautifully - crispy outside, tender inside. The benefit of a thicker fry vs. the McD's type is that they don't cool off as quickly and are therefore edible for a longer period of time. (I find any fries to be pretty nasty once they cool off.) That experience has always had me convinced that you could have excellent frozen fries.
Haven't been to a Burger King in a decade, so I can't comment on theirs in particular.