While not exactly true, it is a smashingly good film.

Rating: 8/10 (9 January 2014)

Before I go any further in my review, I must point out that the film is NOT a 100% accurate portrait of P.L. Travers or the work they needed to bring Mary Poppins to the screen. It is NOT historically accurate in several instances* but overall it is still a very, very good film. Had the film been a bit closer to the truth, I would have scored it higher, as the retired history teacher within me bristles with the notion of changing a few facts in order to tell a better story.
The film is about the rather difficult time the Disney people had in getting the film "Mary Poppins" made. This is because the author was VERY protective of her character and insisted on having full control over just about everything. In some instances this was good but often it seemed that P.L. Travers just wanted to 'win'--such her insistence that the color red not appear in the film! Unreasonable, abrasive and fussy--the film shows the process from start to finish as Walt and his team worked with a very difficult woman to get a classic film made. And, ultimately, it appeared as if the Disney folks just did what they wanted despite her many, many objections (though this is downplayed a bit in the film).
The movie is a wonderful character study and seeing Emma Thompson playing such a strange and emotionally damaged person was fascinating. As for Tom Hanks playing Walt, while his part might seem a lot easier to play (since he was by most accounts human unlike Travers), I appreciated how Disney did NOT make him 100% perfect and sanitized (like many of the books they've produced about the man). In this, he is a bit of a schemer and smokes---making him much more approachable to the average viewer.
In addition, while the film isn't 100% true, it's mostly true and really, really manipulates the viewer (in a positive way). The film, as a result, is quite draining and often sad. My recommendation is that you indeed watch it but have a few Kleenex on hand just in case.
*A few of the 'creative changes' done to the story that differed from reality would include Travers' trip to Disneyland with Walt (which never occurred), Walt's trip to England to persuade her to agree to terms (which never occurred) and Travers loving the film in the end (she STILL hated it and did cry when she saw it at the premier because she hated it so much!!). I know of a few other instances of creative changes but you kind of get the idea. However, overall, the spirit of the film is still pretty close to the true story.