I've never been a real fan of her work, but I have a book on fantasy writing that recommended her, so I thought I'd give this trilogy a try. I think the best part of her writing is the style, and the worst is her grip on the events of her plot.
The book is full of beautiful and fantastic imagery; I'll provide a quote so that you can see for yourself.
"Morgon, I have been thinking . . . I wanted to give you something that might help you; I racked my brains trying to think what, when it occured to me that there are times in your journey that you might simply want to disappear from enemies, from friends, from the world, to rest a while, to think . . . There's nothing less obvious than a tree in a forest."
"A tree." Something in his mind quickened. "Danan, can you teach me that?"
"You have the gift for shape-changing. Shaping a tree is much easier than shaping the vesta. You must simply learn to be still. you know what kind of stillness is in a stone, or a handful of earth."
"I knew once."
"You know, deep in you." Danan looked up at the sky, then glanced at the bustling, preocupied workers around him. "It's easy to be still on a day like this. Come. No one will miss us for a while."
Morgon followed him out of Harte, down the winding, quiet road, then into the forests high above Kyrth. Their footprints broke deep into the powdery snow; they brushed pine branches heavy with it, shook soft snow flurries loose that bared webs of wet, dark fir. They walked silently until, turning, they could not see the road, or Kyrth below it, or Harte, only the dark, motionless trees. They stood there listening. The clouds, softly shaped by the wind, rested on the silence; trees were molded to a stillness that formed the whorls of their bark, corve of branch, the heavy, downward sweep of their needles and the pinnacle of tip. A hawk floated in the silence, barely rippling it, dove deep into it and vanished. Morgon, after a long while, turned to Danan, feeling suddenly alone, and found beside him a great pine, still and dreaming above Isig.
However beautiful, though, the plot of the story moves in violent, shuddering jerks, wrenched out of sensible shape by the fact that nothing is ever explained.
Fantasy literature is full of things that don't actually exist. As such, the words used in it often have a thousand connotations, but they don't denote particular thing. If I say "wizard", do I mean Harry Potter or a world-destroying demi-god or a quack snake-oil salesman? Is a "unicorn" an enormous magical horse or a distant relative of goats? Terms like that always have to be explained, otherwise they remain floating abstractions, concepts detached from referents, a fog filled with vague half-grasped shapes.
Reading Paticia A. McKillip is like floundering through that fog. She takes for granted that you will rely on guesses and vauge memories to fill in the details, but it means that the motivations of her characters never become clear; what they do makes no sense. Instead of the story making the character's actions intelligible, it is the characters actions (and the emotions or erratic thoughts that motivate those actions) that make the story intelligible. The result is that, while you may eventually guess at what the Earth-Masters were or what it means to be a wizard, the characters become irreducable primaries. You can't ask, "why did they act in this way?" any more, because there is no why, meaning that the plot is no longer a logicial progression, but a series of random events that are somehow related.
I find the style extremely eerie and almost physically uncomfortable to read; it's like staggering through a world where any A can become non-A at whim. It's strange how murky waters often appear deep.