My personal opinion of the movie vs the novel is that the movie did a pretty good job of capturing the essence of the novel. Obviously, not everything that happens in the novel was able to be recreated on screen, but I thought the screenwriters did a good job of picking and choosing which events to include.
For example, when Atticus shot the mad dog, I felt the film did a really good job of portraying how scary this actually was. When I read it in the novel, I didn't think it was such a big deal, but when I saw it on film, I realized how this was an epiphany in the lives of Scout and Jem, who thought their dad wasn't really "cool" and couldn't really do much, like the other dads.
I thought the trial scene was good and pretty true to the novel. I also thought the little girl that played Scout was outstanding, as was Atticus (Gregory Peck) and Dill.
The setting is the same in the movie and in the novel - a sleepy little southern town, full of interesting characters, among the bigots. In the novel, the black characters were pretty stereotyped, but when acted on screen, it was more impacting to see the emotion on their faces, which one can only imagine in the novel. The anguish on Tom Robinson's wife's face when Atticus tells her that Tom has been shot is much more vivid on screen.
What did not come across as well in the movie was the excellent irony of Harper Lee's writing. The words she has the characters say in the novel truly reveal her themes of racism, childhood innocence, coming of age, etc., that is perhaps impossible to capture on screen. Her wry sense of humor just did not come through on screen. Some things just have to be read to be appreciated.