There's plenty of controversy surrounding these books. They've been criticized for the apparent incestuousness of the relationship between the two main characters for about 2 books out of 3, for two secondary characters being gay and another a rather promiscuous lady. There's blood and gore and cold-hearted twistedness and frankly I don't think these are YA books. The characters are teenagers, but to me that's not enough to make a book suitable for teen reading. Then of course there was the scandal of Cassandra Clare allegedly plagiarizing everything fantasy from TV to books in her Harry Potter fanfiction. I haven't seen or read the allegedly plagiarized works or the said fanfic so am not qualified to have an opinion on this, I can say though that there are elements in MI that are common to other stories. The extremely powerful and charismatic villain with a group of supporters he gathered while they were all in school and the orphan courageous enough to stand up to him come to mind immediately and I think we all know what they're reminiscent of. Here's what makes that not matter to me: anything after the folk tales, myths and legends is a repeat, anything written now has been done before in one way or another and the important thing is not to create something absolutely original (that I think would be next to impossible) but to make something familiar your own and different. I think Cassandra Clare does that beautifully.
I really enjoyed these books, could hardly put them down really. Ms. Clare's writing is witty, the characters are realistic enough with all their flaws and fine qualities, there's plenty of action and lovelorn adolescent angst to satisfy both types of readers. Even mild character continuity problems, like the bookish Clary who regularly skipped PE in the first book all of a sudden becoming strong and agile in the second, didn't ruin the experience. I loved how developed the world is with plenty of fantastical creatures, their intricate dynamics and how the fantasy world coexists with the regular world with the 'mundie' humans suspecting not a thing. If you are the kind of reader who can manage to not let the things I talked about in the first paragraph make these books unpalatable I think you're in for a treat and you should put them on your To Read list.