I actually liked it better than I liked the book.
That almost never happens with me, with the movies of Lord of the Ring being probably my ultimate video triumph (honestly, I got bogged down in all the Middle Earth history and world-building and such, and characters had such similar names that I couldn't keep them straight when I was reading the books). I had much the same experience with Divergent. They streamlined both the plot and the number of characters, but I thought it helped keep things on track. I thought the biggest weakness of the book was that the characters were a little flat, and since they all had the same dominant characteristic and lifestyle (and there were so many of them), I couldn't remember which was which. Some female name would come up, and I would have to go back and look it up....was that so-and-so's girlfriend? the girl from the fight? the girl from the tattoo place? But in the movie, I could recognize them visually, so I knew who was who all the time.
But I also thought the movie benefitted other ways by being able to show, not tell. I had a much better feel for the different factions, their different ways and energy, in the movie than I did from the book. I also think the movie did a better job of conveying Tris' emotional journey visually compared to the text of the book. I don't want to say anything more and give things away for those of you who haven't seen it, but there were several scenes in the movie I found to be very powerful and that moved me in a way the book hadn't.
The cast, on the whole, is really good. Shailene Woodley makes as good of an action hero as she does a cancer patient, and really, her characters are pretty similar--strong, independent, brave, honest young women. Much more shocking is Ansel Elgort's transformation. In TFIOS, his performance is so exuberant, but then he is so withheld as Tris' straight-laced brother. That's true to the character, so it isn't a criticism--it's just that I like the character of Gus much more than I like Caleb. But then, I like all the characters in TFIOS better than I like the ones in Divergent.
My bottom line is that I recommend the movie to fans of the book as well as to those who haven't read it. It's not a great story, but it is an interesting one, and I think the visual component makes it a more emotional one as well. And it certainly raises some of the issues that are important to adolescents. Where do I belong? Who is my tribe? How much should I follow my family's ways and how much should I be on my own path? Do I trust authority figures or my own intuition? These questions all play out in a very dramatic way, and should at least make teens feel better that their decisions about these questions tend to be a little less irreversible.