I had mixed feelings about that ending.
Obviously, huge spoilers if you read on.
I only read the book once, and it was ten years ago, but I read something online a while ago suggesting the encounter with the Three Kings is far more significant to the story than it seemed when I first read it. From memory, they tell him he has successfully destroyed the Crimson King's operations, he's killed everyone he was involved with, gone insane, and locked himself in the Tower. He's no harm to the Tower or anyone anymore. Okay, I guess you can argue there's an insane superpowerful wizard out there, and Roland might feel a duty to do something about it, but does he? From memory, he's just like "this is great news and all, but I must continue to the Tower". In other words, up til this point, the whole idea that his friends will stop dying if he forgets the Tower was true, but belies the fact that he had a job to do: stop the Crimson King from destroying the Tower. Now the job is done, for sure, and he can go home, but he keeps going. Everything that happens from that point on is collateral that Roland incurs on himself.
I love how the story devolves and decays the closer it gets to the Tower. They leave any noteworthy setting behind, the characters they encounter start to seem ethereal and only half-real. In those final chapters, the last few characters are scrambling, almost on their hands and knees, in the empty waste around the Tower. Roland is being hunted by his son, who killed off the main villain, whose potential power is huge, but is instead just a sick, twisted wraith chasing Daddy through the wilderness, yet, in a Gollum-like way, he's now Roland's worst threat, because Roland is on his last legs and far beyond any aid. That I love about the ending.
The last chapter before the Coda, however, I felt was a letdown. As much as the story's final great duel being this pitiful clash between an almost powerless Roland and the insane KC fits the devolution of the universe at the end of the book, I just hated how it was written. I mean, EEEEEEEEEEEEEEEEEEEEE. We couldn't have had a more developed Crimson King than that? And the way he incorporated Patrick into the ending was a bit too deus-ex-machina-esque. I feel like surely King could have come up with something else that was more satisfying. I didn't really care too much about the Coda and its effect on the narrative, as the ending had already left a sour taste in my mouth.