Flannery O'Connor damned the novel with pretty faint praise when it came out: "I think for a child's book it does all right." That seems unduly harsh (and unfair to YA literature). What works best for me as an adult reader is the slow accretion of local color, the barbed social comedy, and the graceful prose.
Even America's most admirable people were products of their respective cultures. We can learn from their repugnant moments, just as the older Atticus can serve as an example not to follow. But we should treasure the moments when these figures transcended their times and stood for democracy in the broadest sense.