Some thoughts: 
 1) It was off-puttingly shocking at first to read R-rated stuff from JKR, but I got used to it. 
 2) As in her previous books, she writes with a precision and deftness. You aren't overwhelmed with description. Plot builds steadily, and soon there are many, many plates spinning and a huge cast of characters. Each has something to do with another, even if it doesn't appear that way at first. 
 3) Relatedly, as in her previous books, no character or detail is unnecessary. This may seem an obvious writerly thing to do, but I think it's a skill. There is no detail or name that's just there to flesh out the setting; pay attention, because although it may function that way in the beginning, it will have its own role to play later. 
 4) Four and a half stars. I'm not sure exactly why I'm not giving it five stars; I certainly found it incredibly absorbing and hard to put down once it got going. I am still thinking about the characters and their lives. 
 5) OK, now that I have posted my thoughts unbiased, I will go read other reviews! 
 Favorite quotes (hidden within the spoiler tag, though nothing is spoilery): 
 (view spoiler)[ 
 "Her Majesty occupied a very special position in Shirley's interior life." 
 "Very tall, he had a high, balding forehead, and an immensely imitable walk, his arms held rigid by his side, bobbing up and down more than was necessary for forward locomotion." 
 "It was Howard's constant pleasure to open up in the mornings. He loved moving around the shop while the only sound was that of the softly humming chill cabinets, relished bringing it all back to life -- flicking on the lights, pulling up the blinds, lifting lids to uncover the treasures of the chilled counter: the pale gray-green artichokes, the onyx-black olives, the dried tomatoes curled like ruby seahorses in their herb-flecked oil." 
 "Krystal's slow passage up the school had resembled the passage of a goat through the body of a boa constrictor, being highly visible and uncomfortable for both parties concerned." 
 "Every hour that passed added to her grief, because it bore her further away from the living man, and because it was a tiny foretaste of the eternity she would have to spend without him." 
 "A sharp breeze lifted the hems of skirts and rattled the leaves on the immature trees; a spiteful, chill wind that sought out your weakest places, the nape of your neck and your knees, and which denied you the comfort of dreaming, of retreating a little bit from reality." 
 "How awful it was, thought Tessa, remembering Fats the toddler, the way tiny ghosts of your living children haunted your heart; they could never know, and would hate it if they did, how their growing was a constant bereavement." 
 "Life is shortening day and night." 
 "Even though he had expected to feel this way, the pain was not diminished by anticipation, any more than being hit by a train would be less devastating for seeing it approaching down the track; Colin merely suffered twice: in the expectation and in its realization." 
 "Teenagers, Tessa knew, were riven with the fear of ridicule. Those who were without it, and God knew there were few enough of them in the adult world, had natural authority among the young; they ought to be forced to teach." 
 (hide spoiler)]