Until I read this I thought that Hell is the Absence of God was the greatest science fiction story not only by Chiang, but in general. As you might have guessed by now, I was very wrong.
The only thing I can say about this story is that it is beautiful, elegant, sparse in just the right way. This is so far, in my mind, the pinnacle of all of Chiang's works, asking the big questions in just the right way. What the story is, a meditation on existance, an exploration of another world, one vast metaphor, I cannot say. It is all these things and more. The character, for there is only one, is perhaps his greatest yet, as much prop as person and yet so much more.
This is a story that defies all catagories and surpasses all of them. It is my predicition that this will be this years Hugo winner.