It's been a while since I read the books in this series, but watching the movie refreshed my memory on the major events and story. As a whole, I enjoyed this movie, and recommend it to fans of the book series by Cassandra Clare. It will probably be confusing for people who haven't read the books, but there is plenty of humor and action to make the movie enjoyable for movie only goers.
In terms of similarity to the book, this movie did a great job staying true to the spirit of the story, complicated characters and the epic quest Clary takes on. Some of the background history wasn't explained that might or might not have mattered to the film's story. The history of the Clave is talked about briefly, but the mechanics of the institution were not as clear. For instance, they never explained the witch light, or the pacts that the Clave made with downworlders that shouldn't be broken. A sequence of important events happen, one after the other that can feel overwhelming at times, but Simon has a comical role of adding clever commentary that amuses the viewer.
(My favorite was "Oh great. The werewolves are here to save us. . . I can't believe I just said that.")
Coming from a person who doesn't like Jace, the actor warmed me to his character. He is charming and complicated at the same time. The Jace of the film is less cold than the Jace of the books, from what I remember. I'm also glad that they gave Simon some heroic roles because he is actually a really cool character, especially in book two. Magnus Bane was, of course, amazing and I like the way Luke was portrayed.
My only dissappointment was the absence of Brother Zachariah, my favorite character of the entire series as a whole.
To sum it up, this movie is fun, witty, exciting and very close to the book. If you love the Shadow Hunters, you should see it!