It wasn't that bad, but I'll be the first to admit I went in with very low expectations. First thing you have to do, just like with the books, is skip any sense of logic that the faction system could actually work and manage to sustain itself for any length of time. Get that out of the way, and focus on the basic story.
Tris doesn't feel like she belongs. She gets definitive proof from her test that she doesn't. She tries anyway. Cut to endless sequences of her getting stronger and proving to both herself and the others that she belongs there.
In some ways, it works better than the book, but that's because this version of Tris is not the book version. I don't mean just physically. Shailene Woodley plays her a lot softer than the book version, with obvious remorse. Plus, some of the script changes take away some of the things Tris does in the book that made me hate her so much. I like this Tris, and if this was how she'd been written, I probably would've liked her in the book, too.
A lot of the characters are different, actually. The casting of Will and Al is awful in the fact that the two actors look too similar and it's very hard to tell them apart. The actor playing Peter looks way older than the rest of them and never seems menacing. The only one that seemed to nail the core of the character for me was Four (even though he's physically very different than what he thought he looked like from the book). But hey, he is very nice eye candy and actually made me like him as much in the movie as I did the book, so points there.
Structurally, it suffers from too much non-story before it gets to the real meat of it. Rather than try and introduce elements earlier about the Erudite conflict to try and thread the danger throughout the movie instead of dumping it all at the end, they stuck to a lot of how the book was structured (with obvious deletions). It's a big issue with adaptations. A lot of time, they seem to forget that without being privy to the little throwaways we read in the text, we lose that underlying tension that makes the payoff worth it. It has to be done visually if you can get away with it, but that doesn't happen until too late here. The first hour plus is devoted to Tris's initiation, much like the book, and while the individual sequences aren't bad, they didn't feel all that cohesive.
Still. I kind of enjoyed it in spite of itself. Plus, Theo James is so damn pretty, and I do like Four, so that makes up for a lot of shortcomings, lol.