I summed this movie up on Twitter thusly: It's Emma Thompson as P.L. Travers and Tom Hanks as... Tom Hanks with a Walt Disney mustache.
Tom Hanks is one of those actors that never blends in with the character. Same with Julia Roberts and Tom Cruise. They basically play themselves and the other characters call them by different names. Even in Forrest Gump, he was still Tom Hanks with special needs. Under other circumstances, I would be thrilled for this movie (surprised as hell, but thrilled). But that ain't Uncle Walt, no matter how you slice it.
I will probably see this movie and I might enjoy it - but I preemptively hate the decision to cast Tom Hanks as Walt Disney. Just... no.
(I also observed in Twitter that this seems to be the year for movies and specials depicting the creation of beloved franchises - witness An Adventure in Space and Time, which I have been bouncing around the house about that all week).