1) The Susannah stuff bored me a lot. Her place is as a sidekick, not as the main character.
2) The book felt like it's whole purpose was to build up to the climax. Then the end comes, and it's all left till the next book. IMHO, this is the least eventful of all the Tower books. You could sum up the meaningful plot twists in a paragraph or too.
3) Stephen King did the stupidest thing he could possibly do in the story, make it too much about him. I won't spoil it, but if you've read it you'll probably know what I'm talking about. And whats with the ending to the diary at the back? Dumb dumb dumb.
4) Personal taste, but I felt it needed a quick recap at the front like in the other books, because I really can't be arsed to read them all through again to get ready for it.
Basically, this book was just pretty boring to me. At the moment I feel like I'm gonna get Volume VII to see how the story ends, not because I'm gonna enjoy reading it.