Four and a half stars, not rounded up because five is something I'm not extremely generous with.
Anyway, I enjoyed Divergent, but I think I liked this even better. It moved the story forward, answered (or addressed) the niggling issues and questions I had with the world as it was set up in Divergent, mostly to do with whether there was a world out there and that whole faction thing as a set-up in the first place, the romance was angsty without the angst being too obviously manufactured, Tris continues to grow as a character and it didn't have that "middle book in a trilogy" feel.
Well - sort of, perhaps, but not in a bad way, not in a "I have to get this second book written now somehow in order to get from book one to the resolutions in book three", which some middle books tend to have.
I doubt it'll ever make it to being one of those books I'm massively fannish about, but I do think it's a strong series, and especially in this book, even more than in Divergent, the central question of who we are, what do we see ourselves as, how much does our upbringing affect what we are... I'm liking the way that's dealt with, so far anyway, not too much like a a sledgehammer. Oh, and I really like how it's made obvious that the factions, in spite of having certain stronger traits, don't fully define the person - that there are bad people and good people in all the factions, that not even the Erudite, set up as the power-hungry, manipulative villains in the first book and having that judgement re-confirmed throughout the first half of the book, aren't all bad.
I'm very much looking forward to the third book.