I finished Doppelganger in just over a day. The last novel I devoured as voraciously was Neil Gaiman's Anansi Boys.
This is a tight, exciting, tension-packed novel with pacing that forces a reader to plod ever onward. The cultures are well-formed, especially the witches and the Go Rin No Sho-style 5-elemental cosmology, along with other Japanese-influenced cultural bits like the many honorifics, which sent me into pleasantly twitchy flashbacks of my Japanese classes in high school.
The magic is evocative and complex without being overinvolved, weaving the elemental cosmology in with song. Magic's cosmological implications play an important part in the novel, to satisfying effects.
The fight scenes rock all over the place. The fights are exciting, clear, and most of all, I can easily visualize them in my mind. Some novels, the fights come up in my mind's eye as an abstraction, whereas in Doppelganger, I could go shot-to-shot in my imagination.
My only regret is that Warrior and Witch (the sequel) doesn't come out for another half-a-year.
Marie Brennan is well on her way to becoming a name in the fantasy genre.
So, if you haven't already, go and find yourself a copy.