I graded it a C+. It would've been lower if it were not for the performances of Pheonix, who usually does interesting work, and Witherspoon.

"So if we leave behind the "biopic-angle" what the movie offers us, is a story of its own. "

I'm not sure about that. With the amount of musical performances, it felt like a television special; and with the time left in-between, instead of talking heads, we are dealt the usual dramatic events. 

I didn't even realize that the film was directed by Mongold, who did some good indie work early on in his career with Heavy and Copland (or even Girl, Interrupted). His previous two features before Walk were Identity and Kate and Leopold. Not very encouraging.