Well, in response to the jump the shark question, I wouldn't say SK got predictable, because I certainly didn't "predict" that kind of end to such a great series. OK, here's goes an attempt at explanation w/o giving anything away:
During the years between Wizard and Glass and the final 3 books, I got on this kick of picking out the DT references in SK's other books. I got so wrapped up in these "clues" that I kept expecting them to pan out in the final 3 novels. Honestly, I really was expecting to see or hear about characters from MANY of the other books. When Wolves of the Calla came out, I thought it was great, and was really building up to something that would blow me away. I didn't see the Susannah/Mia thing as a repeat of the multiple personality story: I thought it was something different, which goes back to my comment that it isn't what you were thinking.
Then come the last 2 books. While some parts WERE good, and some of it was heartbreaking, the last 2 books felt rushed, and the pan out that I was expecting only came in the form of ONE of the other books (Insomnia). However, this "pan out" made little to no sense AT ALL. It really felt to me that SK was tired of the whole series and put it to rest as fast as he could, and the ending of 7 really suffered for that.
In one of the books it is mentioned that "Ka is a wheel". This does ring true as far as the ending goes, but everything leading up to it in book 7 was a cop out to me, and I was really disappointed when I read the last pages. I hope that the DT comics that are coming out next year will be better than the end of the series.