While Insurgent picks up right where Divergent left off, I somehow felt that I had missed a beat in between the two stories. Whereas I couldn't put Divergent down, I found myself having to pause in Insurgent a few times to gather my thoughts and make sure I knew what was going on. Even then, I still felt overwhelmed.
There's a solid chance that this was all my fault and not Veronica Roth's. I was coming off a massive three-day migraine bender when I read the bulk of Insurgent, and my system was hyped up on the steriods that my neurologist had prescribed to break the migraine cycle. Therefore, I found myself in the weird position of experiencing some of hallucinatory effects that Roth's characters suffered right along with them. This made it very easy to sympathize with them but very difficult to grasp the greater implications of the story.
Overall, though, I think I struggled with Insurgent because the ground kept shifting. Everyone kept double-crossing everyone else. Tris lied to pretty much everyone at one point or another--including herself. Every faction double-crossed every other faction, and every faction fell apart to some extent. Plans were made and fell apart. No plan ever went like it was supposed to, and even the back-up plans and double-crossing plans never quite panned out. And then there were all the simulations and injections and mind-control drugs. It was overwhelming. I spent the entire book feeling like I was on a search for solid ground and never found it. Which is probably exactly how Tris felt--so in that regard, the book was very nicely done; I could absolutely relate to the main character's frustration with her constantly shifting state of reality.
But beyond that, I had a hard time relating to Tris. Her decisions made absolutely no sense to me. While some of them were explained satisfactorily after the fact, for the most part, she came off as a completely erratic, irresponsible thrill-seeker. I understood Tobias even less than her--possibly because he never offered a single word of explanation for what he was doing, yet expected Tris (and everyone else) to be on board with it simply because of who he was. Clearly I would not have belonged in the Dauntless faction. But apparently not in Erudite either, because I kind of loathed Caleb.
I was also very confused by the timeline of the novel; I generally thought that only a day or two had passed and then Tris would make mention of several weeks going by.
With all of that being said, I did still find it to be an extremely intriguing story.... but I did have the feeling that Insurgent was somewhat a necessary middle step between Divergent (which was an excellent novel) and Allegient (which I'm hoping will be excellent in its own right).