When I watch a movie that is based on a book, I don't try to compare it to the book too much. They are two different mediums, and something that may work in a book, doesn't work in a movie. What is important is for the movie to give back the main story line of the book, the feel of the book, and to stand on its own, so that people who don't read the books understand it as well.
The story line I had a problem with. Partly, because I read the book over a year ago, and I don't remember much of it. I'm pretty sure the end was different, but that I understand because of the time. They did commit a major sin though. They revealed a very important plot point, in fact, I think more than one, that doesn't get revealed until the next book. They did stick to it mostly, but still...
The feel was great. Seriously, I came out of the theater pumped up, and that is always a good thing. I saw the world as if I was still in the movie. That may not be that good.
It can stand on its own for the most part. There were some things that I knew from the book, that was a bit problematic in the movie. For example, the people living at the Institute. Who they were wasn't explained well. I would have liked them to be properly introduced. Maybe it was a time issue, or a flow issue, but really, how long would it take to say "Hi, I'm Isabelle, this is Alec, my brother." or something to that effect. We basically don't know anything about anyone, just Clary. All the back stories that they may have is missing. It's implied, but I only caught snippets. That was a negative. The rest of the plot is explained well, can be understood without the book.
The look of the movie was great. The Institute I loved, the demons looked properly hideous, the fight scenes were reasonably exciting. The soundtrack was also really good. I generally don't pay much attention to the music, but that was a huge plus in this one. I especially liked one fight scene music, so I'm definitely getting the soundtrack.
Now on to the cast. I must say, pretty good.
Lily Collins I first noticed in Mirror, Mirror, where I thought how better she was at being Snow White than Kristen Stewart. She was a very believable Clary too. I look forward to seeing her in more movies, I think she'll be great.
Jamie Campbell Bower was properly hot and dark as Jace. I didn't have a clear picture of Jace in my head, but it would have been something like him. I was very glad when he was cast, I always thought he was wasted as the young Grindelwald.
Kevin Zegers as Alec was a bit of a let-down. I just imagined Alec as a lot more imposing figure somehow. A bit like a darker, more muscular Jace. It was especially a let-down, because I always liked Alec.
Some people say Godfrey Gao didn't fit the bill of Magnus. I actually imagined him to be uglier somehow. It was a bit of a pleasant surprise, because I found Godfrey very cute. Therefore, although he didn't fit my Magnus either, I'd rather have him.
Aidan Turner I didn't recognise at first, although I watched Being Human. I knew I had seen him somewhere, but because of all the hair, he was playing Luke, it was hard. He didn't have a major role in the movie, I hope he'll get more story next time. I always thought Luke was a good character.
Casting Jared Harris as Hodge is a bit of a give-away. I won't say more. Spoilers.
Overall, it was a good movie. Watchable. I may even re-watch it in a few months on a bored Sunday. People can take their boyfriends or girlfriends to it as well, and they will probably have a good time too. I do wish it was less show, more story, but the fight scenes were pretty cool. If someone likes the story, they should read the book though. If someone read the book, try to leave your outrage over the little things behind, and just immerse yourself in the sights. That is why this gets 3 stars. Slightly worse than the book.