Look, it's a great book. There's no doubt. Moments of piercing beauty, and sadness. Heroism, and yet full of the real compromises life often requires. And yet....I have grumbles.
Why the insistence on dressing up medieval Spain and the conflict with the Muslim world in fake nomenclature like "Jaddites", "Asherites", "Kindath" - I spent much of the novel going "right, the Kindath are the Jews, and - wait - were the Asherites the Muslins or the Spaniards?". I don't see that attempting to move this novel away from historical terms, while the setting remains intrinsically bound to the cultures described, really buys anything. Sure, the events don't align entirely with real history - but so what? That would not bother any of Kay's readers. I think the novel's power would have grown had he more directly leveraged the known dynamics between the cultures. I know I as a reader would have spent less time on mental gymnastics trying to map the terms to the historical analogs I know he was evoking.
On more than one occasion something fundamental would happen to one of the characters, a death, injury, what have you, and Kay would refuse to name the character, obfuscating to whom the mortal blow had been dealt. In one important case, I understand why (avoiding a spoiler here). But in the other cases, I found these "cliffhangers" more annoying than suspenseful. I can always skip forward a few pages to figure out who, but why toy with the reader, especially in the early to middle parts of the book? More annoying than suspenseful.
Kay always leverages "archetypal" moments to create power. Here, the love triangle, the yearning for a married man, the duel to the death, ethnic cleansing, the warrior/poet - these are powerful thematic elements. But sometimes the moments feel contrived - a few times I could almost feel Kay reaching for the place where the archetypes are kept, to pull one off the shelf in order to move the plot forward. But it's a small complaint.
Kay finds ways to evoke the culture of a place. In Ysabel, he found a way to evoke the south of France with power. I think in Lions, he found a way to evoke the courtly obsession with honor and country in Spain, and the sensitivity to disrespect - there are strong echoes here of Perez-Reverte, particularly the Captain Alatriste works. (This is not to imply Kay is the lesser writer at all - they are both writers I admire greatly - but the tone and evocation are eerily similar to me.)
In any case, grumbles aside - read it. You won't be sorry.