There'a a reason the first 4 books of Dark Tower are all fantastic - he wrote them years apart, and only when he felt the urge. Then, after his car accident, he started to feel mortal, and decided to get them finished before he died. As a result, books 5 and 6 are a bit tedious, not as memorable or powerful. On the other hand, book 7, the finale, is one of the best books he ever wrote, and that's saying something. Books 5 and 6 are not terrible at all, and they're certainly worth getting through for the epic book 7.