I guess I will start by saying, I read both of the books (both around 500 pages) in a little over a week, so I obviously enjoyed them. They were exciting and were hard to put down. At the same time, they were definitely not as good as The Giver, Fahrenheit 451, or even the Hunger Games Trilogy. It's hard to describe why, exactly, other than to say they didn't make me feel as deeply and didn't make me pause to think as much. Also, at times, the quality of writing and choice of words wasn't as excellent as the aforementioned books.
I just realized that perhaps one reason I didn't love the books is because I didn't feel the society was entirely believable, I don't feel society can be put into merely 5 factions...but I suppose the fact that it is beginning to fall apart in the books shows the author does not think it can be divided as such either, at least not for long.
One thing which was nice is there was not really a triangle of love, as appears to often happen in recent books written for this age group.
What are the books about? They are about a damaged “faction” (not really a class or caste system, because one is not necessarily above another) based society in which members take an aptitude test at 16 to help them choose which faction they will join. They then must go through an initiation process. Some do not make it through initiation and become one of the “Factionless,” the outcasts and, if they do have a job, it is a job no one else wants. Some die during the initiation process, especially in the faction of Dauntless, which one of the main characters chooses to join.
The books are about discovering who you are, what you are made of, learning to face your fears, the difference between bravery and stupidity, selflessness and a sick sense of atoning for your wrongs by putting yourself at risk and allowing yourself to be hurt. They are about friendship and love, learning to trust and realizing people are more complex than they seem. They are about a perceived threat and how it can motivate people to act in different ways, cause them to betray those they love for what they view as the right course. They are about the question concerning “the greater good” versus the value of a life. They are about truth and information, how some think it best to “protect” people by keeping them in ignorance and some think people should know those secrets which can break apart your life and make the world a scary place.
Are they worth reading? Yes. Are they favorite? No. Am I looking forward to when the third one is coming out and will I re-read the first two before it does (it should be out the end of 2013)? Yes. Also, the fact that the author is only like 23 and wrote most or all of the first novel while in college is pretty amazing.
What would I rate them? PG-13 for violence, disturbing situations, some smooching, and a little language. Though there is a lot of violence, I would still say it's a step down from the Hunger Games books. There's nothing quite as gruesome as the Tracker-Jack scene or the Dogs-with-the-human-eyes (by the way, the movie tamed both of those scences down).
Side-note: In Insurgent there are a couple blatant inconsistencies which, unfortunately made it past the author, the editors, and the beta readers. The author is, of course, now aware of them and I'm sure will change them in future printings, but for now...you can have fun by finding them yourself. So you won't drive yourself crazy through the whole thing, they are near the end.