At first I was dissatisfied with the endings.
Then, I thought on it some more, and the more I thought about it, the more I'm really convinced that it's not really the end. I mean, King isn't going to write anymore books about it, but it's not really the end of the story. Let me explain.
For one thing, the title of the ending is Coda. A Coda isn't just a second ending, but a recapitulation of a piece of music, often with slight embellishments or changes. Listen to one of Sousa's marches for a prime example. In the same vein, the story repeats itself, but things work out a little differently. It even starts differently - now Roland has the horn.
It also mirrors the theme of redemption. What lays at the top of the tower is redemption. All of the character's doors reflect this, and so Roland's does as well. In his case the redemption is unstated, though hinted at often in the books - he needs to learn to trust his friends, and forgive himself for his past. With the horn, he lives up to the last promise that he made his best friend. In fact, one could argue that he has redeemed himself in forgiving himself, and thus he no longer feels like he failed Cuthburt, as well as the others.
We're also told that one can not go back in time in the prime world. However, we can infer that this is not totally correct, because in order for Roland to repeat his story, his must do exactly that. My theory (and it's only that) is simply that the door at the top of the Dark Tower acts as a "reset" button of sorts, turning back time altogether. The person that resets it somewhat immune to it - they lose memories, but what they take from the tower and more importantly, their journey to it, stays with them.
Thus, we know that the story stays sort of the same, but it's still all the journey that matters, which is what King says before the Coda.
Two other small points - one, on the prime world Susannah's father knows she's missing and with Roland. Except Susannah is from a non-prime world, as we're told they all are. Second, I don't buy that Susannah would simply accept a new world as her own, and I think based on what we know of her that she would eventually try and roam the worlds again.