I sat on reviewing this for a long time. I have been trying to discern how I really felt about it. The up swell of hate this has generated isn't really something I can understand. If anything, I feel that the second book was the "bad" book in this bunch. I'm glad that it got moving semi-quickly in this book.
The faults I have found with the series is it basically reiterates itself over and over again, with slightly more dire circumstances. The confusion of the dual narratives is something I understand complaining about. They had the same voice and without differentiation it just felt a little pointless. After Four's narratives, he would show up and tell Tris everything that happened...which is something that could have been done without being taken away on the 14th rebellion against x, y, z.
This book doesn't fall flat, but it does pale against the first book. I never felt as captured (although I did feel more involved) as I did in the first book.
All in all this book provides the emotional depth that the 1st book couldn't and all the character development that the 2nd really should have had.
Spoilers, like in the first sentence
In answer to did she just kill Tris for shock? I don't feel that she did at all. I picture it this way: In action movies people dodge thousands of bullets and have infinite ammunition against their enemies, or infinite deus ex machina. People die in war and people sacrifice in rebellions; I guess I'm saying I appreciate Tris' sacrifice.
The one thing it robbed any character of is Caleb's redemption, which to be honest was clunky to me. No one just offers themselves up after 2 lines of dialogue.
I recommend this series, as it is entertaining and interesting. I just pray people can make it through Insurgent.