First off I should mention that I have not read the books, but I've heard mixed things from those who have. Some liked it and some didn't, which is about normal. Personally I expect book adaptions to stray from the book so as long as they entertain me and don't butcher the books, making a sequel impossible (like with Eragon), I don't mind changes. But like I said, I haven't read the books so no comparison will happen.
The Mortal Instruments is labeled as fantasy, but to me it felt like fantasy combined with supernatural. We had supernatural elements like vampires, werewolves, and demons added to fantasy elements like magic, witches, and warlocks. We also have new terms like shadowhunters, which are half-angel warriors who protect the world from demons. On top of this, we are in New York City, but all these fantasy/supernatural elements are in a city right inside New York City that is hidden from the common eye. And before you start comparing this to Twilight, stop it. Yes there are vampires, werewolves, and teen romance, but that's where the comparisons end. And it didn't really feel like Harry Potter either. In fact, it reminded me more of the TV show Supernatural than anything, with added fantasy twists to it.
Our story line centers around a girl named Clary who starts seeing weird symbols and people that her friends can't see. Shortly after she is horrified when she sees one of these people only she can see murder a person, who later she learns was a demon and the man was a shadowhunter. Following this event, her mother disappears after their apartment gets ransacked and Clary learns that she is in fact a descendant of these shadowhunters and her mom has just been getting her memory blocked and so she joins these shadowhunters and thus enters this dangerous hidden world.
I thought this story was brilliant. It was very suspenseful and very entertaining. The world itself was visually amazing as well. The demons were insane and awesome and so were the shadowhunters, werewolves, and vampires. As far as the cast goes, I thought the movie was well cast. No, the acting wasn't always superb, but it was good enough for most of the movie. Lilly Collins was our main star and she was great. Her eyebrows still bother me, but outside that Phil Collins' daughter is quite amazing. Her character went through the most progression out of anyone and I was impressed with that. Outside Lilly, the rest of the cast is fairly unrecognizable. The biggest name for me was Kevin Durand from the TV series Lost who is excellent at playing the idiot in whatever he is in. Other main characters are played by Jamie Campbell Bower, who showed up in both Harry Potter and Twilight, Kevin Zegers, who was the kid from Airbud, and a few others that no one has heard of, but they all did a decent job.
Yes, there were flaws. I don't mind romance in movies at all, in fact I expect it and even love triangles can be done well despite the fact that Twilight has put a sour view on them, but the romance in this is just not done well and the movie would've been better without it. Specifically there is one scene with Clary and Jace that is done so bad it is painful to watch. Also they had this idea that Bach was a shadowhunter and learned to discover demons by playing certain piano chords and that to me was just dumb. Just be classic and do the holy water thing or something like that. There were also a few other minor things, but overall the rest of the movie was good enough that it made up for these parts.
In conclusion, if you like fantasy and supernatural themed books and movies, I would suggest giving this one a shot because it deserved to be a huge summer blockbuster and deserved to have the whole series turned into movies and now it has failed so bad that the second one, which they have already started to make plans for, probably will never happen. This means that in order for me to get more Mortal Instruments like I want, I will be forced to turn to the books. As far as the movie goes, I give it an 8 out of 10.