There are a few spoilers in this review, but I don't think they're major.
I enjoyed it a lot; it was a good, interesting fantasy with a rollicking plot, excellent characterization, and a lot of nifty steampunky worldbuilding, including a truly gigantic steam-powered computer and couriers that fly with metal wings, assisted by a magic anti-gravity metal called ondium. I also very much loved all the class issues that were addressed.
The city of Ondium is built on a mountain, and the winged couriers, or Icarii, are the fastest way to send messages up and down (they also have wireferries, but many people just have to climb). The society is based on caste, reinforced by a belief in reincarnation. Icarii are somewhat outside this caste system, presumably because they must travel freely and deliver messages to all strata.
The heroine, Taya, is an Icarus. After she's in the wrong place at the wrong time (or the opposite!) she becomes involved both with some people from the highest class, the Exalteds, and tangled in investigations to unearth a terrorist plot against the government. She's from the laboring classes originally, and has a lot of ambiguous feelings about dealing with Exalteds throughout; eventually she has to make some painful decisions. One of her Icarus friends is opposed to the government, so he also offers some counterpoint on these issues.
The two other major characters are brothers, both Exalteds, but one of them has a lot of political power and the other has caused a scandal by fleeing to a lower level, where he becomes a clockmaker/repairman. Both of them are really interesting characters, and though I could easily predict what would happen to one of them, the other was more problematic. Their fate and their relationships with Taya kept me interested all the way to the end.
I was especially impressed with the way the author presented flying with mechanical wings from Taya's pov. I don't know if I grasped all of it, but it sounded technical and realistic. The giant computer was much more fantastic, and I could easily imagine it being rendered in CGI.
Overall, it was an enjoyable adventure novel with both romance and mystery elements.
As a side note, I think it might make truly excellent airplane reading, given its length.