The problem with this film is also its most interesting asset -- the filmmaker. The film sheds little light on Louis Kahn's character or his architectural abilities, and it says basically nothing about architecture whatsoever, so if you are looking for a film about architecture, move along.
Beyond a great number of shots of Lou's sometimes beautiful, sometimes unpleasant buildings, this film is not about architecture.
What it is about, and what it excels in portraying, is a man's search for a father he lost in his youth and with whose ghost he has not yet made peace. Nathaniel Kahn has not made himself a very likable character, and for that I suppose one must respect him. He wavers between cloying innocence and childish sullenness and smugness. I think especially of his near falling-out with his mother. He comes across as downright cruel in not allowing his mother her idealizations and delusions about Lou's intentions toward her. Watching this film, the viewer does see the damage losing a parent does to a young child. Nathaniel is stunted and boyish, and sweet in an altogether unlikeable way. The Nathaniel we are given in this movie haunted by his father and his inability to understand him or to resolve his feelings toward him.
Little Kahn does have a number of interesting interviews with the people who knew his father; it's an interesting study of how greatly people are affected by a single person, how disparate their recollections of the person are, and also, how similar they sometimes are too. Apart from a few biographical facts, this film could have been about anyone who was greatly loved and deeply complicated (i.e., about a quarter of the people most of us know). Did it need to be about Lou Kahn to succeed? No.