Absolutely fantastic!
I've pretty much covered what I like about the story, as well as Zahn's work overall, so I won't repeat it other than to say again how much I dig the fact that it is a very adult novel. No cutesy bullshit, no droid antics, no children's comedy. A serious science fiction story continuing in the vein of the first two films, but with much deeper character development. Mara was particularly well written.
Truthfully, there's no way this wouldn't make a great film. I was visualizing it as though I were actually watching it. I also dug that 3PO is a background character and R2 is just a machine, not some larger than life character who flies (honestly - what the fuck, Lucas?).
Anyway, I finished it up a few hours ago and will be starting on Dark Force Rising just after I hit post reply.