This review contains information that may possibly be considered spoilers by some people, but in my opinion is vague enough not to warrant hiding it under spoiler tags.
Insurgent is more focused on action than its predecessor, Divergent, which necessarily had more character- and world-building. Personally I like character-focused books, so I didn't really get into Insurgent as much as I did Divergent.
I wrote a lengthy rant in my review of Divergent about how ridiculous it was to set up a society this way in order to foster peace. I still think that is the case, however in the last few pages of this book we learn that the system seems to have been set up to develop Divergents - people with "flexible minds", who are supposed to save the rest of the world, which has presumably plunged into chaos because they all the people in it are "inflexible". A society in which every single mind is totally rigid seems equally ridiculous (not to mention biologically impossible) to me, and I don't have much hope that Roth is going to come up with a satisfactory explanation as to how the world "outside the fence" came to be that way.
I also take issue with the idea that "human nature" is the ultimate problem to be solved, and that "human nature" can "become" something that is anti-human and self-destructive. In my opinion, it is the systems that humans set up for themselves that can become destructive. Humans lived in balance with each other and nature for hundreds of thousands of years, and it's only our present culture that is destructive. Human nature is not the same as human culture. So I guess what I'm saying is that I totally disagree with the fundamental idea of this book series. Which makes it difficult to review this book. Unless, of course, the ultimate lesson at the end is the point I've just made. It's possible Roth will surprise me and come to this conclusion, so I have to withhold final judgement until I read the third book.
Setting aside my foundational issues, the story is fast-paced, and never boring. Like many readers, I did get annoyed with Tris for not being able to get over herself. She became a lot more like the stereotypical YA protagonists with her emotion-driven recklessness and guilt mongering. Plus, I really didn't get why Four/Tobias wouldn't even discuss going after the information. His reasons were never explained, and it makes no sense in terms of his character. Truly, it seemed like a totally contrived conflict to create tension between the couple. And man, I really hope Caleb can explain himself, because I'll be super angry at Roth if she really turned one of my favorite characters into an immoral sack of crap who'd do that to his own sister. It doesn't make sense in terms of what we have been led to believe about his personality.
I honestly can't give a final verdict on this story, because so much that I find questionable may or may not be explained, rectified or redeemed in the next book. I save my ultimate conclusions until then.