Thoughtful and terrifying and compulsively entertaining. This is what the science fiction genre was made for. 
 Taking one of the most drastic hypotheticals, the most direly destabilizing of test cases, LeGuin sets about addressing the ambiguous and subjective nature of reality (in many ways, she out-PKDs her contemporary Phillip K. Dick, here) in direct but subtly calibrated ways, spinning off deep and troubling tangents worthy of their own stories as barest afterthought. Then she uses her sanely-insane apparatus to closely interrogate manifold ethical problems: the individual's responsibility to society, what exactly the greatest good is and can be, whether any individual is even capable of deciding such things. I've noticed other reviews here mentioning the "implausibilty" but this matters not a bit in service of such broad goals as here. Seriously, this is the kind of bold and insane premise that I wish much more sci-fi would dare to tackle. It's the book's best asset. 
 Though, as I said, it's also relentlessly gripping from the start, opening well into its action and then expanding uncontrollably upon the premise without stopping, wasting no time or words, often revealing world-detail elegantly in asides and background description. On the other hand, it's pace, concision, and scope demand a kind of obviousness at odds with some of my deepest-held sensibilities. Though at the time I was so caught up that I barely noticed, and behind the obvious bits, there are subtler shadings of idea flitting about, deepening the picture effectively. 
 I read this right after The Other Side, another very different, though also strangely compelling, vision of reality-warping dreamstate. In some ways, by broadcasting its questions more clearly, this is the far less dreamlike and ambiguous of the two, though it probably makes the stronger case for itself, ultimately. 
 As M says "I'd always been more into the idea of Ursula K. Le Guin" (we both read some of the Earthseas decades ago, independently, remember being somewhat bored, know that we should revisit), but this bears out Le Guin's promise quite spectacularly. Recommend me other essentials of hers in this vein, please! 
 Final afterthought: Jonathan Lethem has clearly read this / borrowed concepts and inspirations at least twice.