I went to the movie knowing it was based upon a book series, but that was it. I spent much of the next two hours jumping right out of my skin.
The storyline is sort of a mixture of the Harry Potter series, Twilight, and a little Star Wars thrown in. There is definitely a lot of supernatural action and magic, with demons, witches, angels and vampires battling - but no real, regular humans. There are also werewolves, and there are people of mixed blood who work to save the rest of us from the bad guys (apparently all the scary stories told to us when we were young are true).
Our 15-year-old New York City heroine, Clary, wakes up one day to find she can see things others cannot see, that scary bad creatures are after her, and her life is definitely not what she had ever thought. She is a critical player with secret powers in an invisible war between good and evil. Plus, she is the only one who may be able to find the "Mortal Cup" the bad guys want so they can control the world.
Before the day is out, her mother has told her not to come home and has disappeared, her home is trashed, and Clary is attacked by a vicious demon/serpent/dog and is running for her life. She then meets a group of "shadow hunters" who are actually folks with angel blood saving the world from demons and other unsavory characters. Warning: In this story the bad guys are not always bad and the good guys are not always good - you really don't know who to trust. Pay attention.
Clary isn't really a strong heroine until the last bit of the movie - it takes her a while to pick up the threads of everything. The actors do a decent enough job with their parts, though some of the dialogue was unintentionally funny. At one point, another character is told he has a different identity than he has known about, and he is told "you are a Morgenstern." If you are old enough to remember the sit-com "Rhoda", this might make you giggle.
Another bit of lore in the movie tells us that Bach was a shadow hunter and that a particular song of his was written mathematically and when played, it causes demons pain. That was a bit over the top for me.
I have to say I was entertained because I had no idea what was going to happen next. The nonstop action with swordfights, symbols and runes carved into skin bringing magical powers, demons being able to take over people's bodies without them knowing it, and so on are coming from the screen hard and fast.
My companion, who has read the books, hated the movie, and national and regional movie critics universally agreed and panned it. Critics who read the books say that some scenes and storylines have been altered and much has been omitted from the movie. More often than not, however, regular people are enjoying the movie, from what I'm reading.
I think it depends upon one's expectations. For a bit of entertainment, a person who has not read the books might have a bit of trouble following some of the details, but if it's action and some stolen romance here and there in a supernatural setting you're looking for, then go see the movie. I give it two and a half miners.