Fallout is not-quite-history, not-quite-fiction. It's the story of the making of the atomic bomb, with actual quotes and documents, featuring the actual scientists involved. But it's also a graphic novel, with some people removed and some quotes used in a different context, to make a good story that's a little easier to follow. As the title suggests, it deals with the politics involved as much as with the science, and gives special treatment to the case of J. Robert Oppenheimer's hearing after the war was over.
I'd read a few books about the atomic bomb in college for a class (John Hersey's Hiroshima and Richard Rhodes' The Making of the Atomic Bombwere two that I remember), so it wasn't an entirely new subject. However, it did provide some insights into some of the behind-the-scenes bickering that went on. Also, I know I'd read a little about Oppenheimer's hearing, but this book filled in some details about both the hearing itself and its findings.
The combination of different artists gives each section of the book its own feel. Towards the end, there's a lot of text used in sidebars, which makes for awkward reading. However, it does have an interesting effect, reading both the majority and minority reports from the hearing next to each other. One thing I wish I done was to follow the endnotes while reading the story: Ottaviani provides copious notes about sources of quotes, when a quote actually was said by somebody else, and other interesting facts. He also includes a long list of references which would make a good reading list about World War II and the bomb.