I'm not going to repeat what plenty of people have pointed out regarding the glaring holes in the worldbuilding. There's no denying the central concept of the factions, which underpins the whole plot, stretches believability to breaking point. 
 However, once you put that aside, the story is fast-paced, well written and a lot of fun to read. The characterisation is good and there's plenty of action. It's a shame the process of Tris' initiation into her new faction was the sole focus of so much of the book. Not that it wasn't fun to read about, but it made the "real" plot feel a little disjointed when it did arrive. I would have preferred if there had been more foreshadowing & build-up during the earlier parts of the book, to link it in better. 
 Once you get to the main action, though, there are no punches pulled. The challenges and danger faced by Tris and her allies, have real consequences, and where it would have been easy for the author to back away from them, she does not. 
 I also enjoyed the romance subplot, especially because it steered clear of the insta-love / thinly-veiled-stalker tropes in a lot of YA at the moment. Unlike some reviewers, I found the emotional connection believable, although there were some moments when one or other character acted inconsistently without good reason, for example 
 (view spoiler)[ If Four was worried about showing his affection for Tris because people would think his favouritism affected the initiation rankings, why was this suddenly not a problem a few minutes after the rankings had been announced? Did they think everyone would believe there had conveniently been nothing between them during testing to prompt their kiss in full public view afterwards? (hide spoiler)] 
 Overall, I would say it's worth overlooking the worldbuilding flaws for an entertaining read, and I'm looking forward to finishing the trilogy because once the main plot is revealed towards the end of Divergent I can see the potential for a whole lot more excitement ahead.