It looks like most reviewers have this pretty well covered, but I'd like to throw in my 2 cents, mostly because I've spent so much time reading these books and it just plain feels right.
I'll start by saying that this book is not perfect, and there were times when I was convinced that the story would end horribly. So why do I give it 5 stars? I feel that what was good or great about the book is enough for me to dismiss what annoyed me about it. As with most reviews, there will be many SPOILERS ahead, so go read the book first if you haven't.
I can cover what truly bothered me pretty quickly; King's importance in the plot and Flagg's death. Not that I need a 150 page battle between Roland and Flagg, but to be so easily dispatched by a child who would later prove to be absolutely no threat to Roland seems wrong. Was this really the best that Roland's lifelong enemy could do? I don't know, it just doesn't add up. As for King, I didn't like his part in the plot during Song of Susannah (narcissistic) and was hoping it would not dominate the final volume, which it doesn't. I will admit that it didn't turn out as bad as I feared (I half expected Stephen King to be in the top room of the Tower), and I do appreciate that it ties the story to reality through something other than the fictional Tet Corporation in NY. While this is nice, I could have done without King ever having been in the story.
Others were bothered by the quick deaths of Mordred and the Crimson King. I wasn't. Mordred may have been powerful, but he was still a child; and a sick, dying child at that. He had to make an ill-advised move out of desperation, and I think it was wonderful that Oy was able to die defending Roland. As for the Crimson King, while the battle may have been brief, it was still exciting and not without great word play between Roland and the CK.
Ok, so most of us want opinions and theories on the end, right? I will admit that I very much liked the ending from the get-go, but I have had to digest it for a few days before really knowing what to make of it. I suggest anyone else that has recently finished the book do the same. In doing so, I have realized that this ending has evoked a lot more emotion and thought from me than I expected, and clearly almost everyone else who has read this book has had the same experience, for better or worse. In my opinion, this alone is proof that the ending is very powerful.
So, let's just say it, we find out that Roland has been repeating his journey from the Mohaine Desert, where we first meet him in book I. Why is this a good ending? It puts the entire story in perspective. Roland, while heroic, has been cold hearted throughout the saga. He seems to be learning to love, particularly in the case of his adopted son Jake, as well as the rest of his ka-tet. Would we have ever guessed from the pages of "The Gunslinger" or even "The Drawing of the Three" that Roland would have such an emotional reaction to Susannah finally leaving him at the end of the story? Probably not. The world had moved on, a world that had been "full of love and light", and the gunslinger that we meet in book I is a hero, but can't be described as "full of love".
During this journey, Roland has accomplished his purpose according to prophecy, which is to save the Tower. I personally believe that he has probably accomplished this in the past journeys, but it doesn't really matter. The Tower, it seems, is more than just an anchor for existence; it is holy, and will judge those who enter it. When Roland arrives in the Mohaine once again, he is given the Horn of Eld. This can signify many things, but I believe that it signifies that he has pleased the Tower and this will be his final journey. King appends Browning's poem, which has Roland blowing the "slug horn" at the end of his journey. It could be that Browning's poem actually IS the final journey, meaning that the end of Roland's quest is actually right there for you to read about, or it could just be a vision of what his final journey will be like. I think it can go either way, the point is that we know that Roland will have peace, and soon.
Need proof? Let's look back to Jake's final death, a sad scene that will bring even the most cynical reader to tears in light of life's harsh truths. In this scene, Roland promises that he will sacrifice himself to save Stephen King rather than Jake, and his decision is made when he jumps from a moving vehicle to do just that. We all know how it turns out, but the point is that the gunslinger has changed since first letting Jake drop into the abyss in book I. I believe this, along with the care he gave to the rest of his ka-tet toward the end of the journey is what finally pleased the Tower, which awards him with the Horn of Eld. When Roland arrives at the Tower in Browning's poem, the spectators (roses in King's story) in the final stanza are "met to view the last of [him]". I think that King interprets the last of Roland as in the last time that he will arrive at the Tower. After all, King does state earlier in the book that there is "only one place for the poem", which is at the end, as the poem reveals Roland's final journey.
I don't want to make light of the sadness that comes with this ending, because it is certainly sad. The hope combined with sadness and the endless possibilities are what makes this ending so brilliant. I think I've covered the main point, but perhaps Roland has still more to learn and atone for on his final journey, and there are other things to consider. Did time rewind (which his watch rewinding as he approaches the tower and the re-installment of the Man in Black would suggest) or was he simply placed back in the desert? Will he meet Jake again on this journey? Has he learned enough to save Jake rather than palaver with Walter? Are Roland's enemies (Flagg, the CK, etc) so desperate to kill him because they are caught in his loop and aware of it? Could the Horn of Eld symbolize the line of Eld, meaning that Roland is to save Jake thereby restoring "love and light" into his world (and also ensuring that his son will continue in the line of Eld) before the Tower will let him rest? I could go on and on with these, and some of them may be reaching, but I think that if you believe that the Tower has finally begun to award his progress during his damnation then you can believe that he will be able to right all wrongs.
I will miss reading these books, I hope the rest of you take as much from it as I did. Long days and pleasant nights.