I absolutely love this series. I do have a confession though. I've been stalled out in the middle of Wizard and Glass for a couple years. I'll read it the rest of the way through before the new books come out though.
My favorite so far is definitely The Wasteland, with the Drawing of the Three being my least favorite in the series. My reason is that the first book was extremely cool, but his writing style was so unrefined back then, I mean it was his first project, that it was hard to follow at times. The Drawing of the Three was also good, but what I disliked about it is that too much of it takes place in our own world. What I like best about the series is that otherworldly feel, and the notion that the world itself has somehow shifted and changed. This is somewhat lost in The Drawing of the Three, but I find the writing style is a little easier to get through than the first book. The Wastelands is great because we're back in Roland's World, or at least what's left of it, SK's writing style is excellent in this book, and the story is captivating. I'm reserving judgment on Wizard and Glass until I've actually read all the way through it.
I can't wait for 5 - 7 to come out. I was beginning to think that SK would kick off before getting these finished. That would have been a let down, but also the most intriguing body of work from a person that can only be described as a very prolific writer.
I can only hope that the dark tower books are as good as it. SO far I am not disappointed 
