I think the film version of the story visually frames the end of the book in the most appropriately poetic way it could have been done by Harper Lee's ending. The final scene takes what is otherwise a story made up of a series of memories and flashbacks, and ties them together through the adult Scout's eyes. For the first time, the entire neighborhood, and all of the inhabitants, are scene as parts of a whole. Everyone has a purpose, and everyone's role is shown as important in the society in which Scout grew up (and perhaps important to Scout's development more than anything).
I love that the trial scene is not in fact the climax of the book. I love the opening and closing of the novel around the most unlikely character of all, Boo Radley. I love that "mockingbird" in the end, is not just Tom Robinson.
I think there are a number of transformations that the ending of the story highlights. Firstly, Jem and Scout have been transformed from being young and innocent to worldly and experienced in some senses. They have definitely changed through what they have seen and witnessed. Secondly, Boo Radley is transformed from the bogey man and object of mockery to the hero of the story, and Scout realises that it is wrong to judge and objectify others. Finally, Bob Ewell has moved from being a bad force to being deleted from the story.
It also shows the conflict Atticus faced when trying to raise his children to believe in the goodness of a world in which he clearly saw the ugliness. Yes he tried to teach Scout that "everyone is nice if one really looks at him or her." At the same time he's watching Jem recover from the vicious attack of a man who clearly was not nice no matter how many chances he was given or no matter how long you look at him.
I think it's an appropriate ending. Boo has been seen, and he becomes the hero of the novel. The evil Bob Ewell is dead, and Maycomb is safe. Scout's fantasy about seeing Boo has been fulfilled, and she understands that most people are "nice" once you get to know them. Life can get back to normal in Maycomb, and life can now go on for both Jem and Scout.
I look at two things from the very end of the book -- Atticus telling Scout that everyone is nice if you look at them, and Atticus keeping watch over Jem as he slept. I think she ends the book that way because these are two of the major things she has been trying to explore in the book. She has been looking at Atticus as he tries to help his kids grow up as good people who are not prejudiced against others. The end of the book shows this. It shows Atticus and Scout talking about accepting others and it shows Atticus's devotion to his kids.