According to IMDB, Margaret Mitchell approved of the film (overall) and liked the job Vivien Leigh did as Scarlett. Here's their quote: "She personally approved of the interpretation of Scarlett O'Hara by actress Vivien Leigh when she finally saw Gone with the Wind (1939). Overall, she also liked the film with a few minor quibbles. She thought the house at Tara (the O'Hara family plantation) should have been much more modest. She also reportedly did not like the introductory "crawl" that appears on the screen after the opening titles ("There was a land of cavaliers and cotton fields called the Old South ... ")."
I've both heard and read in the past that Margaret Mitchell wanted Leslie Howard to play Ashley Wilkes and was pleased that he did so. (I didn't think he fit her description, but she did evidently. I suppose I have chutzpa!)
***As a note to dana2592: Why Hollywood didn't attempt a sequel will be answered by this bit of trivia - also on IMDB - "After her death, her brother Stephens Mitchell controlled the copyright to "Gone With the Wind". During his life, he steadfastly refused to grant rights to any sequel (print or film) or filmed remake. Only after his death was the sequel approved, which was adapted as the TV miniseries "Scarlett" (1994)."
***Note2: I didn't like Scarlett, either, but I might "bite the bullet" and read Rhett Butler's People. I approach sequels (or additions) to classics and written by somebody else with great trepidation! Yikes, I really hope today's Hollywood doesn't have the temerity to think they can do a better job than the original movie! The possibility of who they may cast as Ashley, Rhett and Scarlett is mind-boggling. A copyright is, from what I understand, in existence for a full 70 years after the author's death. If the author died after January 1st of the year, the copyright exists until the January 1st following the 70-year mark (at least in America).