First off, let's get this out of the way: this is NOT a book about the ethics of human cloning; nor is it (in any conventional sense) "Science Fiction". Not that there is anything wrong with Sci-Fi: I've read and enjoyed a lot of it over the years. However, this definitely isn't it - it has much more in common with Kafka than with Philip K. Dick.
Ishiguro's tale is both moving and sinister from the start, and gets increasingly so as it goes on. In a darkly dreamlike Parallel England, a self-styled "ex-student" at what initially seems to be a boarding school deep in the country is recounting (in a deliberately flat, almost Enid-Blytonesque style) the childhood experiences of herself and her best friends. However, Ishiguro makes it abundantly clear from the first couple of pages onwards that all of the "students" are destined for a sticky end: indeed, one of the main points of the book is that the students are fully aware of their eventual fate even from a young age. They understand this information on a factual level, and even make crude jokes about it, but they have never properly internalised the full implications. For this reason among others, they passively accept the inhuman horror that awaits them.
For me, Ishiguro clearly intends the book as a sort of dream-parable to say various things about the human condition in general. Firstly, if we grow up with a horror (nuclear weapons, say, or Third World poverty - Ishiguro silently invites the reader to make his or her own list), then human nature is to take it for granted as an immutable Fact of Life and just accept it. The eventual fate of the Hailsham "students" is one that no sane person could possibly endorse: and that's exactly the point. (There's no "Ethics of Cloning" debate here - it's surely an open and shut case - and Ishiguro deliberately leaves the science of what is going on very sketchy.)
Many aspects of Kathy's tale are true for all of us. Like her contemporaries at Hailsham, we all know that we will inevitably die one day, and nothing - not True Love, High Art or whatever - will make one blind bit of difference. What, then, is the point of it all? Ishiguro's answer is initially a surprising one: "the little things" - the small change of human friendships and kindnesses; a favourite T-shirt; a "special" song off a second-hand cassette. This is what ultimately makes Kathy's tale so heartbreaking, and what makes the book ring so true emotionally.
At a key point in her childhood, Kathy describes a chilling realisation about one of the teachers or "guardians" as being like "seeing something strange and unexpected in the corner of a mirror you walk past every day" (I don't have the book to hand at present, so apologies if this isn't quite word-for-word.) This could well stand as a superscription to all of Ishiguro's fiction from "A Pale View of Hills" onwards, but is particularly appropriate as regards this unique and extremely unsettling book.