I have to agree with most of the people here.... POV shifts can work, but they need to be planned out, and in most cases, brought in from the very beginning. Normally, sudden shifts of style late in the book annoy me, too.
But like just about everything else, if you can pull it off, it can work. The example that comes to mind is The Help. (Not fantasy, I know, but I couldn't think of an example in the fantasy genre). Most of the book is in first-person, jumping between three different narrators. Then, there's a climactic scene at the end that suddenly shifts to third-person omniscient. Yes, it's jarring.... But that's kind of what makes it work for me, because of the context. You need the third-person POV for that scene to "work".
It probably also helps that in the case of The Help, the technique is used very sparingly. There's nothing else particularly unusual about the way the book is structured, and it's only a one-chapter diversion before returning to the original style that the author had established. For me, it was effective.