I admire Patricia McKillip's dialogue style in this trilogy. It's somber, cerebral, and quiet. (Note that I didn't say the narration; McKillip doesn't sound like Ernest Hemingway.) I have studied it, even, because as an aspiring novelist, I would like my characters to sound somewhat like McKillip's if possible and if appropriate.
I think the Riddle-Master Trilogy is stronger in characterization than in plot. There was always a quality of limited detail about the events, the antagonists, McKillip's fictional world. I mean that as a neutral observation; I don't necessarily dislike it. But the characterization is always more vivid than any aspect of the plot or setting. It's the opposite of Lord of the Rings, really, and creates a sense of mystery that trilogy lacks.
I liked the plot well enough, but how complex and original is it? Without reading many more fantasy classics, I'm not sure; but I'll say this: about halfway through the series, I correctly guessed how it would end, based only on details provided on the back covers of the three books.