I'm not able to conjure up the violent hatred of some of my GR friends who've reviewed this book, but I think that's because I've only read the first book of this series. I've heard Jace and Clary had a nice resolution at the end of book three before being gratuitously pulled apart again here, but I didn't experience it and I'm not irked about it. 
 I will say Jace is not as vibrant here as he was in book 1. I kind of liked him there; I thought he even showed a bit of complexity in being so arrogant on the one hand and so utterly cowed by his father on the other. His wisecracks come off a bit stale here, a bit tired, and yes there is the dire wangst noted by other reviewers. 
 Simon's story -- which strikes me as far more interesting than Jace/Clary -- is reasonably interesting but takes the back burner too much. It had a lot of potential, but I feel we're only ever skating along the surface of it. 
 I didn't mind the plot as much as some people did, but then plot is not the thing I really read for. I thought Lilith's destruction made sense, even if it was a little obvious, and I thought the final scene was perfectly comprehensible -- again, to the point of obviousness -- so I'm not sure why there's been confusion on that point. It's not a subtle or well-thought-out plot, and it depends very heavily on people not communicating well (especially at the party when Simon goes missing -- NOBODY thinks to ask Clary [his super-duper-best-friend] if she's seen him? Oh, right, because then they'd notice Clary is missing too!). Also, the idea that Lilith, the Grande Dame Demoness du Monde, would be engaged in experimental genetics, essentially, strikes me as kind of silly. All these different kinds of blood, with different powers? Gotta collect 'em all? It didn't make a lot of sense, and you get the impression that the author herself couldn't tell you exactly what was going on. 
 I didn't like how the action was defused again and again and again. Any time it seemed like something interesting was going to happen, we'd cut to another POV. Then, when we get back to where the impending action was, we'd get a snapshot of the character in an unexpected position and a FLASHBACK SUMMARY of the action that just happened. Or else you'd get characters discussing the action that happened somewhere else. It was all off screen, just about. 
 Another thing that irritated me was the way there would be flashbacks (or sometimes just random thinking about stuff) in the middle of dialogue, such that the dialogue would be really difficult to keep track of, to say nothing of any emotional impact it was supposed to have. Simon (for example) would ask a question, then there would be 2-3 long paragraphs of him thinking about something else, and then Jace would answer the question but his answer wouldn't make a lick of sense until you turned back a page and found the question again. I'm going to assert that this is an ineffective use of dialogue. 
 So why the two stars, and not one? Well, two things: 1)it gave me something to do on the way to Alaska and back, for which I am grateful, and 2)I found it kind of instructive. It helped solidify my understanding of WHY it's important to show action instead of just telling it, WHY having too many POV characters can undermine the impactfulness of any one character's story, WHY meandering thinking in the middle of dialogue is distracting, etc etc. It was an object lesson in shooting yourself in the foot, and I appreciated that. 
 And I thought there were places the book succeeded. There were a few times she built up atmospheric creepiness enough that my seatmate's hand on my shoulder made me jump. Simon being unable to stop himself biting that girl was well done, to my mind (then immediately turned farcical by the arrival of both his girlfriends -- oh well). Jace's nightmare, where you can see what's going on and he just can't, was nicely horrible. I thought the very last scene was creepier and more vibrant than anything else in the book. Now, maybe other people are better-versed in horror than I am and can point out that these scenes were derivative or something, but I thought they were ok, and if they're representative of Clare at her best, maybe Clare needs a little editorial encouragement to stretch herself in those directions.