I just finished the series last night (say hallelujah, say amen) and although I am still processing it all, at this point I do know these two things:
1) As others have said, Ka is a wheel and the ending was so beautifully reflective of that. I agree with those who feel it could not, should not have ended any other way. I felt it was brilliant.
2) During my reading of the series, there were so many times when I would shriek with frustration when King would write that Roland or Eddie or Susannah or Jake knew something, but they didn't know how they knew it. I thought - at the time - that King was being just downright lazy and simply didn't want to do the work that was needed to explain. But now, now I feel differently and I need to cry Sai King's pardon. As we now know, Roland will be making his journey to the Dark Tower yet again (and perhaps again and again and again, and has probably already made that journey countless times already), so when he knows something but doesn't know how he knows it, it's most probably because he learned it or discovered it on one of his previous journeys to the Tower. And I feel the same goes for the rest of our beloved ka-tet, that the reason they know something (but do not know how they know it), is because they, too, have learned or discovered it on a previous incarnation of the journey that they are destined to continue to take with Roland until...well, that part, I don't know, but I suspect as so many others do, that Roland needs to get "something" right. Now, I don't know if King intended it that way, but I'm so in awe of his storytelling genius right now that I'm more than willing to give him credit for it.
I apologize for the long post, but I just must say one more thing...thankee, Sai King, for an unforgettable journey that made me laugh and cry and yell and swear and hold my breath and forget to make dinner and sit on the edge of my seat and feel like I was part of Roland's ka-tet...thankee big-big!