Imagine the Harry Potter books being compressed into 1 film, with no Ron, Hermione or 5/6 years of Hogwarts; just one film very loosely based on the Potter/Voldemort war with 90% of the characters and plot missing. This is what has happened here.
The Dark Tower series is one of my favourite of all time. Each book is better than the last, and usually I find that not to be the case with series. Whilst I do like Stephen King books, I'm not usually that enamoured of his stuff. However the Dark Tower books and the way they tie in with his other work is absolute genius. I knew that one film could never match the books, or come close. I was expecting to be disappointed and I was. If I hadn't read the books I may have found it OK I guess.