A generation of girls grew up with them, even though they were adult books. I don't know what I'd feel if I read them for the first time today, but they were fascinating to me at the time. I think it's because the main character in the Flowers in The Wind books had the emotional development of a tween -- and stayed there. When you're a kid, there's something compelling about a character who operates at that same level.
It was Southern Gothic, but read very much like a fairytale -- the brave children, the betraying mother, the scary "witch"...