There seems to be a surplus of teen friendly movies based on books.
If you combined Twilight, Harry Potter, and Buffy the Vampire Slayer in a teen friendly Twilight-want-to-be film, you would end up with The Mortal Instruments: City of Bones. Do I need to say any more? I found the film interesting because I like Buffy The Vampire Slayer, but the movie and the characters don't feel exceptional in any way. The film has enough to hold my attention, but it relies on action and fight scenes a little too much. It lacks the really great characters that would make something like this wonderful. The television show Buffy The Vampire Slayer had more interesting characters than this.
That isn't to say that there weren't some cool things in the show, but I would like to see a sequel that makes the characters more appealing.
The most common criticism of the movie is that it feels recycled. The Mortal Instruments: City of Bones garnered only 12% approval on Rotten Tomatoes, but I feel that the movie is not so terrible. If you like shows like Twilight, Harry Potter, or Buffy The Vampire Slayer, then this is a three star film. For everyone else I give it two and half stars. If you hate these kinds of movies, and chances are there are many people who do, then skip it.