It makes a great effort to stick close to the facts, but like any film, it takes some short-cuts (to improve narrative flow, dramatic effect, etc...)
There are some good articles on Wikipedia (and the references there will give you a great deal more to read, if you like):
http://en.wikipedia.org/wiki/Operation_Valkyrie http://en.wikipedia.org/wiki/July_20_plot http://en.wikipedia.org/wiki/Valkyrie_(film)
