While Marlon Brown is a very athletic WR, a five star ranking and top 20 national ranking is too much. I know he got a lot of press after he had 300 yards receiving in one game.
I haven't seen one of his football games in person, so obviously I don't know as much as the experts, but having seen him play basketball, his speed and quickness are not what you would expect from a five star WR.
His size, strength, and great hands will help him become a good player, but like a few people have already said, he is by no means an immediate impact type of player.
Unfortunately, a number of fans will call him a bust after his redshirt freshman season when he doesn't produce Robert Meachem like numbers. But give him a few years, then he will begin to match his ranking.
Also, from my perspective, Marlon Brown is not going to be a big factor on the basketball court. His 30 points in the state championship came on all lay-ups and free throws, not fancy drives and perimeter or even mid-range shots.
Also, while the people who rank the prospects are obviously smarter than me, I don't see how someone like Austin Long (who I go to school with) is so highly ranked and someone like JerQuari Schofield is not ranked. Long has good size and decent strength, but I don't see him making an impact at Georgia until his redshirt sophomore or redshirt junior season.