(Spoilers, if you haven't read the book, but it's been out for over a year, so I won't feel too bad about this)
He (and most of his commenters) really liked the ending, thought it was the perfect way to end the series, and so on. Not me. I'll freely admit than when I read the ending, my first thought was "Well, ok, what happens when he gets to the tower this time?" It felt to me like an unresolved, tacked-on ending that was artsy for the sake of artsy-ness. But, since I often miss the point of artsy-ness, let me try to explain myself, so that maybe someone can tell me when I'm missing the point.
Anyone who's played a video game has had this experience- you go though a giant dungeon, spending hours killing monsters, almost dying, using all of your potions, finally you get to the last door with the boss behind it, your get ready to kick down the door...
Crap. I forgot to find the key. Back though the dungeon, fighting through monsters, spending hours, trying not to die (if I don't just reset and start the dungeon over), only to get back to where I was.
This happens to me most often in Legend of Zelda ("what do you mean I need to go get the Master Sword?!?"), but it's a frequent enough occurrence that I hope everyone knows what I'm talking about.
Anyway, back to the book. I got the feeling that the last scene was exactly that situation. He gets all the way to the tower, but forgot his special key, and so has to restart his entire game to pick up the key and then do everything all over again.
(Real video games, of course, wouldn't be that mean- of if they were, we'd stop playing. Just like I don't want to restart Kingdom Hearts all over again and do everything "right" just to get the good ending. But the story is meant to be life, not a game, so I can forgive that).
My main beef with the ending is that we know Roland is going to go through all of the same stuff again, and we know that Roland is eventually get to the tower with "all of the right stuff" (if not this time, then in some future attempt when he has the "Magic Hat of Exxon" or something), and we know that when that happens, he'll actually get to the top of the tower and get the "good ending". So by leaving the story where we do, we don't get to see the end. So why show us this failed attempt, when it would have been just as easy to show us the subsequent successful one?
And if you're going to say "well, because it's all about the journey, and this is an infinite loop", or whatever, then let me ask you this- isn't everything that you think is good about this ending just as good-- if not better -- if he doesn't pick up the horn, and goes back to the exact same starting point?
If he doesn't pick up the horn, I may agree with you about the ending. If he alludes to all of the previous trips and says "boy I'm glad I got the horn this time", then the door closes behind him, I'd be ok with that ending (but not happy- I want to know what's going on, dammit!). But as it stands, it just begs the question- what's going to happen when he gets to the tower next time?