The 2011 re-read:
Susanna/Mia uses Black Thirteen and flees to New York to have her baby. Roland, Eddie, Jake, and Callahan get the Manni to open the Unfound Door and end up in the wrong places. Can Roland and Eddie convince Calvin Tower to sell them the lot where the Rose grows? Can Jake and Callahan find Susannah before she has her baby?
Song of Susannah was my least favorite book in the Dark Tower series the first time through. Susannah has never been my favorite character in the Dark Tower saga and this book is really Susannah-heavy. On the second read, I had to raise it another star. The Susannah/Mia conflict had its moments and did a lot of setting up for the big shebang coming in the seventh and final book. A lot more background information was revealed.
I'd be lying if I said Susannah was the reason I upped this one a star upon re-reading. The other reason is that Jake and Eddie seem like bonafide Gunslingers in this book, even more than they did in Wolves of the Calla. I have a feeling their fates in The Dark Tower are going to be almost as painful the second time through. The relationship between Eddie and Roland has developed quite a bit since The Drawing of the Three, as has Roland's character. I still love Long, Tall, and Ugly, even though he'd probably leave me along the Path of the Beam the first morning I bitched about not having coffee.
There is something else that I liked a lot more the second time but it's pretty spoilerific. Now that I've had a few years to digest Stephen King writing himself into the story, it doesn't really irk me like it did the first time.
While it's not my favorite of the Dark Tower books, it's still good and it lays a lot of the cards down on the table for the final volume, The Dark Tower. See you all along the Path of the Beam.