I will say that this was more entertaining than TMI series to me, but I still had a few problems
SOME SPOILERS BELOW
I wasn't a huge fan of the writing/plot of TMI series, but I've heard this was better so I thought I would give it a shot. I do think that it's a better start, but there were too many similarities for my taste.
As others have mentioned out, Clare recycles plots and characters like crazy. For instance, Tessa is a bookworm version of Clary, Will is obviously Jace, Jem is a mixture between Alec and Simon, and Jessy is a bitch so I won't compare her to Isobel (who is much better in my opinion). Someone (brother/mother) goes missing and Shadowhunters at the Institute (London/New York) help the girl who isn't just a mundane to find them and uncovers people breaking the Shadowhunter laws.
The difference is, I actually liked the characters in TMI more. Tessa is a bit bland, and because of the time period of the setting, very proper. She just seemed a bit too clever and accepts everything much better than someone with her ability would. Jem is the GEM in this one (ha!). He's thoughtful, pleasant, intelligent, and of course, dying. Will is pretty much exactly like Jace. I'm not sure what his past is, but I doubt it excuses his behavior. I don't really know why Tessa is drawn to him at all. Jessy is a MONSTER. Seriously, she has no redeeming qualities I have no idea why she's even there. I loved the Magnus cameo, Camille not so much. Oh, and then there's the case of the evil brother. Clare must have had a seriously messed up brother or something, because everytime someone has a brother (or thinks they do), they turn out to be evil or crazy.
Then there's the plot. I totally understood being afraid of the Magister when everyone thought it was De Quincy, but the reveal makes absolutely no sense. Why would all sorts of Demon creatures be afraid/take orders from a mundane? I guess that's the point, no one sees it coming, but that's because it makes NO SENSE. De Quincy could have ripped his throat out in a second, but somehow he's afraid of the guy? Nonsense.
Overall, it was okay. Hopefully the next book is in a different direction. I am glad that she didn't make us this that Tess and Will and/or Jem were siblings though (small victories).