I just finished Song of Susannah and I must say I'm very impressed. I definitely think King fell off the wagon for book 5, but he got right on back (and then some) for book 6. The ending is a big cliff hanger which is to be expected as he's done that before. I'm a little weirded out by the author's notes/journal though. They all seemed to be a little too perfect.
Any way you look at it if you like the Dark Tower series you'll definitely like Song of Susannah... wow.