This book has me slightly puzzled, because now I've managed to escape from its clutches and stop checking for... no, to tell you what it made me scared of would be a spoiler... anyway, now I've got "out" of it, it's hard to analyse what made it so gripping.
We have an ancient man-made (er, sentient-made) space city, all luxury on top and slums in the sewers/recycling mechanism. Nothing new there, standard cliche of either SF or fantasy. Tribe culture in the underworld - check. Gritty references to food and water from recycling - check. Of our two heros, one is a youngster in the underworld who has recently lost his protector, the other is a wealthy newcomer to the entire world who can therefore receive explanations that also explain things to the reader. Check, check. (Though I notice that many of these things are cliches I'm more used to meeting in fantasy than in SF). There are other cliches later on, too, mainly about alien powers and what the climax consists of, but I'll refrain from spoilers.
The Angels of the title are state-sponsored assassins. Now, that's new. This is a democracy by assassination: for a politician, failing is a very bad idea. You may "win" the vote to be Removed. We get a look at the concept from the viewpoint of victim, of assassin, and of audience. It's well-thought out, it's different, and the consequences have been thought through.
What else is different? Well, that young hero is a male prostitute - by choice. For once, this subject gets treated in a refreshingly non-hysterical fashion.
I think what gripped me, though, was the characterisation. Both heroes, and many of the other characters, are sympathetic people. They're not idiots, but they do have human flaws. The action keeps moving at a good pace, enough to stop me putting the book down without getting breathless. Because they feel real, the threats seem real, and because much of the background is undemanding, familiar, stuff, you don't get distracted by it.