It's very much out of date, like most works from the early 20th century. It's also highly ideological, a work of rhetoric by one of the last centuries masters of that art. If you read it, read it for the literary qualities of the work. Churchill's other histories ("The World Crisis" and "The Second World War") can be read as primary sources, since the man was a principle decision maker with access to important archives. Like all primary sources, corroboration and context are required to consider any assertion in Churchill to be valid.