I don't watch movies from books where they don't follow it. I never read Lord Of The Rings, or any JR Tolkein, so the movies were fine.
I did read Gone With The Wind...that movie was spot on.
I did read The Green Mile more than once...and it was spot on.
I have read and reread The Dark Tower. Roland is not black and the drawings King had inserted in those books show Roland as craggy caucasian...so...in my mind, that is what he is. White. The only reason they made him black was because of being pc, I'm betting. And for that reason, I refuse to watch. Just like I would not watch a book character switching races.
Your own mileage may vary.
"Go then. There are other worlds than these"~Jake
Idris Elba is big right now.
So far as I'm aware, Eddie and Susannah are not in this movie at all. By comparison, Roland being a black man is a trifle.
Nick Fury has been a white guy for many years. Unlike Roland, Nick Fury is a character that is normally portrayed in a visual medium. Despite that, I'm OK with Samuel L Jackson playing him in the movies. Sure, it took some getting used to, because I had seen the character portrayed as a white guy for so long. It still worked out fine.
Besides, Roland's ethnicity is not integral to his character. I don't remember anything in the books which made his being white important to his character. He could be black, Asian, latino, it wouldn't really matter.