So I read two of the books. Then saw the film. That may have been a mistake.
The film does bear a resemblance to the book, but it may well be easier listing what they got right than what they got wrong (Most of the first third was accurate enough). Went to see it with Ailsa, Tara and Suzanne, and Ailsa was immensely unimpressed – but she’s geeking out massively about it; it’s entertaining to watch.
The film was enjoyable, but it doesn’t follow the book plot except in the vaguest sense. They reveal things from the second and third books, and added whole new parts cut whole cloth that weren’t even in the book, as well as redeeming one of the villains.
Still, it wasn’t a bad film – it just wasn’t the book. It’s likely going to be the next Twilight in terms of films (and audience), but has several things up on Twilight. For a start, the main character is actually pretty badass and doesn’t just wilt in the presence of danger and wait for her hero to rescue her. And vampires are mean, like they should be (as well as being beautiful creatures of the night).
I would probably recommend not seeing it with a bunch of impressionable teenage girls – they may shriek at the sight of Jace being badass.