The good points: Ace was more along the lines of the series Ace, Benny was still funny and smart, there was a reference to One and Susan (I am beginning to think I'm a sucker for that kind of thing), and some of the underlying plot made sense.
The bad parts: everything else. Good lord, this book was a slog to get through. Crispin was completely unbelievable, Forgwyn even more so. Ace being back along the lines of the series Ace was good for me, but bad for the character... there was no way to explain how she got back there from where she's been, character-wise. The dystopian setting was straight out of the time period without anything to distinguish it. I couldn't have cared less about the Friars of Pangloss.
It's really very sad, because I can see what could have been a good book under there. Seeing an older woman who is what Ace could become might have been good for our hera, but it's never explored. Benny could have been better used, although her decision to "go exploring" is right in character. A robotic Doctor is a great idea. Robots taking over major media figures is classic, and could have been so much fun. The Vijans are touched on, then never used again. Ever. What a waste.
The rest of the book is just awful, though. Crispin's about-face is reasonless and as arbitrary as they come. The slaags are more grotesque than scary. All of the bad guys are copied from pantomime. They just needed mustaches to twirl. The organized crime boss could have been interesting but was given incredibly little to do. The way scenes are telegraphed leads me to believe that this was started as a script with stage directions and the writer wasn't good enough to make the shift.
It's really sad when you find yourself thinking "At least no one walked up and started declaiming about what Tragedy Day really is." Bleah.