Yes, the characters are fairly two-dimensional. Yes, there is a stupid insta-romance which I could happily have done without (although "Four" is a likeable character and not too bad as far as protagonist's-love-interest go). And yes, reading this requires total suspension of disbelief. There are definite, obvious flaws. And yet... overall, I enjoyed this. I can definitely see its appeal on the mass market, and there was enough substance to keep me reading. It helped that the writing isn't bad. This is no great work of literature, but Roth's words are enjoyable and simple to read, which is no detriment to her.
I agree with those who wish this were classed under something other than dystopia - if there had been more emphasis on the world itself, then maybe the dystopian label would make more sense. There were mentions of needing to patrol the fence, but that was quickly forgotten (hopefully to be brought up in the next book) and the actual state of people's lives was hardly mentioned, other than a few comments about living in poverty and a general lack of wealth. These things may be remedied in the second and third books.
One thing that bugged me more than it should was the train-jumping. Maybe I just have a terrible grasp of physics (I hated it at school) but how slow would a train have to move for a large number of people to jump off at slightly different times (staggered over about a minute) and still land in exactly the same place? When I think of trains, I think of extreme speed, and it just didn't make sense. I could be totally wrong, in which case please ignore this whole paragraph!
It may also be worthwhile to note that while reading this book I kept feeling an odd sense of deja-vu. Where had I read this before? It hit me just now that something in this reminded me very very strongly of Uglies by Scott Westerfield. I loved that book when I read it (far more so than DIvergent, actually, though that could be down to age - I haven't read Uglies for years) and but there are definite similarities between them (the trains, the thrill rides, changing life age 16, the protagonist's insistence that she is ugly etc etc).
Oh, and the ending was terrible, but not enough to dissuade me from reading the next book.