While the main premise of this story (everyone divided into factions: Abnegation, Amity, Candor, Dauntless or Erudite and then basically being discouraged from having contact with the other factions, even if your family or close friends belong to them), is pretty ridiculous and unrealistic, there was something about this book that just grabbed me and did not let go until I finished and eagerly waited to obtain the sequel.
I'm not saying the writing was great, because honestly I don't normally notice that kind of thing unless it's obnoxiously bad, but I had no complaints in that regard.
The pace and tone of the story kept me hooked. Every time I read it I could feel myself tensing with excitement and found myself working to conceal those emotions because I normally read in public.
I generally avoid young adult because I don't like the direction that this genre has taken lately with paranormal everything and romances with guys who would be completely creepy obsessive in real life but its ok because they're so hot but this is one I recommend. Of course there's a romance (how could there not be) and yes Four is a bit "dark and mysterious" but before all his secrets were revealed, I had no trouble figuring out why he was distant and acted the way he did. His motivations in trying to keep Tris safe seemed clear to me, even when Tris was clueless and thought he was being a jerk.
All in all, if you aren't nitpicky and can suspend your disbelief, I recommend giving this one a try. I really enjoyed it, and read it in a day.