Almost never do I watch a movie and then decide to read the book that it was based on. Because, sorry to say, movies are usually not decent "versions" of their books - they fail to do anything different in terms of the story or the character development or the settings. I'd say that's what the movie is for.
I watched the movie first - in 3 installments on DVD, which is really the only way to watch it because I don't see how anyone sat through the 3-hour movie in one sitting. With such quick shifts between characters and time periods it required concentration, which wasn't unwelcome since each story arc was intriguing in its own right. I applaud the choice of the movie in switching quickly between arcs - it gave us a better sense of the connections between each of the stories. In the novel, each story arc gets two sections, layered as if it were the earth - if you started on one side of the globe and traveled through the earth to the other side - or as if the center of the earth was the future and the crust is the past. That works quite well too - in novel form - because you don't have to "catch up" to understand where you are.
Mitchell works hard to show his heart on his sleeve, in good fiction writer fashion, by pointing out the evils of everything ranging from branding to nuclear safety to genetic manipulation of plant life. Each story has a different tale to tell about a particular part of the human experience. While you'd think (and hope) that the inner story would have the message of hope, he does not quite give that to you - leaving you with a feeling of dread about how we live and an urgent sense of disaster if we don't fix some things right away.