I’m going to be lazy with this one, and quote other people. In the pro camp, Alvaro Zinos-Amaro:
And now for something quite extraordinary. If you’re looking for a single reason to purchase Eclipse Two then you may be out of luck, because Ted Chiang’s “Exhalation” is at least three of them. It is without a doubt one of the finest and freshest breaths of story I’ve ever come across. It is immediately compelling, a superlative example of a story that pulls on a seemingly mundane observational thread, and reasonably proceeds from it to unwrap the entire fabric of existence. Though time will be the ultimate judge, I have no reservation in calling it a masterpiece. The story is narrated in the first person by a character who appears to make a crucial discovery about air. What he is able to deduce from his initial and further experiments comprises the narrative’s unforgettable journey into expanding consciousness. The story’s execution, on the whole minimalist in approach, is flawless. It unfolds in gradual revelation, and every component fits as perfectly as those described in the character’s empirical forays into literal self-reflexivity. Of course, it also functions superbly on a metaphorical level and pays tribute to classic SF stories dealing with entropy and thermodynamics. The intellectual thrill of reading it might be compared to directly experiencing William Blake’s “world in a grain of sand” and “eternity in an hour,” except in this case contained in a few molecules of air.
Rich Horton also likes it:
“Exhalation” is quite as spectacular as last year’s Hugo winner, “The Merchant and the Alchemist’s Gate”, and yet completely different. It depicts an utterly unusual artificial world, apparently completely made of metal, whose inhabitants are likewise metal, and who breathe air supplied by replaceable lungs. It is told by one of these people, who discovers how their brains work, as it becomes clear that the supply of air is diminishing. The setup seems to imply some history that other writers might have exploited — is this a society of robots after humans have left, perhaps? — but Chiang’s interests are elsewhere, and the story explores deeper philosophical questions, and comes to a very moving conclusion. To make the obvious pun — it took my breath away.
Ted Chiang’s “Exhalation,” though a chilly thought exercise of a story, is a chilly thought exercise by Ted Chiang and therefore cooler, more inventive, and more interesting than just about anyone else’s chilly thought exercises.
But Martin Lewis is less keen:
If you had told me before I had read the stories that I would be rating the Chiang bottom I would have told you to pull the other one. Generally, it is much as you would expect a Chiang story to be: typically rigourous, taking a single idea and working it through. Unfortunately it is a lame idea. Chiang sits us down and explains the terrible beauty of, er, entropy. Great. Oh, and it contains no dialogue which must make it slipstream.
EDIT: And Ian Sales:
It is a truth universally acknowledged that Chiang is one of the best writers of short science fiction currently being published. Which means every Chiang story is not only judged against all others published around the same time but against every other Chiang story. Which does him no favours. Especially in this case. ‘Exhalation’ is pretty much a thought experiment, with very little in the way of plot. It’s well-written, but it failed for me in several aspects. It lectures the reader… and the explanation for this doesn’t quite justify the up-front info-dumping. Further, the central premise isn’t actually that interesting, and all the story does is provide a slow and cumbersome vehicle for the narrator to figure out that entropy exists.