Being a massive Dark Tower series fan I am coming into this movie with Ultra-Low expectation. The story is just too complex and detailed to ever even have the smallest hope of being well done in movie format without turning it into a trilogy with 3+ hours on each movie, and this is at bare minimum.
After seeing the trailers I am both very disappointed and impressed with the direction of the film. I am disappointed by how they are trying to turn it into a Sci-Fi action film that literally takes place, in part, on other worlds and not other dimensions/timelines that the books take place in. I am heavily disappointed at how they show Roland having supernatural speed and pre-made revolver bullet clips and what seems to be an unlimited supply of munitions including both loose bullets and revolver clips. I am impressed, however, with how the casting of the characters and how, at least in the short clips, the personalities of the characters are true to the books.
This story is neither a sequel, nor a prequel, and is certainly not canon to the books either. The Dark Tower movie, IMHO, is best described as a Roland and Jake story based loosely on the Dark Tower Series world and events.
The writer and directors has forgotten the face of their father.