MAKING OF A LEGEND is one of the great documentaries about filmmaking. Few movies had a more interesting production than GWTW and the documentary did an excellent job at covering it. I'd go as far as to say I think the story behind GWTW is more interesting than the movie itself. If memory serves, Olivia didn't participate in the documentary which might be its only obvious gap; as the sole surviving main cast member, her involvement would have been beneficial. (Her own documentary, MELANIE REMEMBERS, isn't nearly as good.)
As for GWTW itself, I admit I have muddled feelings about it. I generally have no problem putting works of art in the context of the time they were made, but GWTW is so deeply rooted in and responsible for perpetuating an inherent lie -- that moonlight & magnolias, noble Lost Cause BS that the South has pushed for 160 years -- I find it an increasingly bitter pill to swallow. It's not unlike a grand romance set in 1930s Germany where the heroes are all Nazi-sympathizers. Beyond that, I think the movie fails for the reason mentioned: Leslie Howard is terrible and I find it impossible to believe Scarlett would moon over that mawkish pansy when the alternative was Clark Gable.
Which isn't to say I find the movie bad. It's a remarkable production with some absolutely gorgeous visuals and is, at times, an emotionally charged melodrama. Vivien Leigh's florid performance is one of the best and practically carries the film on her shoulders. And, yes, Mammy is the only character in the movie that I actually like. Maybe she's a racist stereotype, but she's sensible and salty; I think I'd rather watch a movie about her.