The best book in the series. My favorite part of it -well, of the series as a whole- was the idea of how everything we know could turn out to be a lie. Over and over again. And the detail about GP people believing there hadn't been wars before the Purity War, that GP people couldn't start wars. Because it showed how they were manipulated by the government into believing false historical facts true. It reminded me a little of how history was dealt with in Orwell's 1984. If you control the present, you control the past.
Like always, there were things I liked and things I didn't like, but I think the one that bothered me the most was Evelyn just changing her mind about everything she fought for and believed in during Insurgent
just because of her son. I mean, it was quite out of character for her. I would have loved some actual development which made her take said decision.
And, as much as I wanted Tris to live and be happy with Four, her death didn't bothered me. Actually, I liked it. I guess I'm too much into GoT...
I loved having Tobias' POV, and I think Roth made a wise decision adding it. Tris' fate apart, having another character's POV allowed the reader to experience the whole picture of what was happening. However, I also think Tobias' voice should had been more distinctive - it sounded too similar to Tris' sometimes.
His character development, on the other hand, was pretty good.