It's a daring authorial gambit. Just as the plot is gaining momentum, King stops the main story, the one his loyal readers have been impatiently following since 1982, dead in its tracks and indulges in what at first feels very much like a 550-page digression. Taken on its own terms, it's a well-crafted romantic adventure, albeit one that could benefit from being shortened by about a third. What makes the flashback so frustrating is that, even as it illuminates interesting new nuances of Roland's character, it ultimately seems like one more detour on a journey that's already taken far longer than it should. It's a hodge-podge of incidents, characters and conventions that really shouldn't work together but that mostly do, mainly through the sheer, stubborn force of the author's storytelling prowess.
Readers can either go along for the ride or get the heck out of the way.