I liked it a lot. There are already many comments on Internet (thanks Russ for making me discover these novels), so I will not go into details. And it is hard to summarise without spoiling. In brief:
The first tome, Ancillary Justice, makes us visit various worlds and cultures, and give us an impression of what it feels to be a demigod. The main culture does not make a difference between the two sexes, and the grammar of its language does not have genders. This gives an original taste to the story, for instance when the hero speaks a foreign language, he has difficulties to correctly address people without risking to frown them. Unfortunately the English language itself does not use gender very much, so the literary effect is a bit weakened. Perhaps the French translation (which I have not read) could be more interesting in that respect?
The second tome, Ancillary Sword, shows us how one can communicate things in a surveillance society without privacy, by subtle variations on how to serve tea. Gallons of tea are drunk in this tome, of which the main interest is the relation between the characters and heir conversations.
In the third tome, Ancillary Mercy, asks the question of what makes us human. Among the most interesting characters, there is a kind of synthetic human, who acts as ambassador for an alien race. At first, he indeed behaves completely alien, but in the end, he is not very different from a newborn who would happen by miracle to know how to speak: in the beginning the World makes no sense, but step by step and by experimenting, he deduces how it works. This is how this character ends up understanding that what is called "war" is a complex phenomenon where one of the consequences is a shortage of fish sauce.
I was a bit surprised that no book lead us at the heart of the Radch empire, but I just see on Wikipedia that one more novel is in preparation... One can speculate that central Radch resembles to a future dystopian West, in which surveillance of everybody is total and constant, but where people think they are happy, and peace and well-being inside are kept possible thanks to military operations outside, mostly performed by killer robots controlled by artificial intelligences. A not so distant future ?
It is a matter of course that there does not seem to by any Free software in the Radch empire. That reminds me that I did not contribute much to Debian while I was reading...