I've never been more than a casual fan of the series, so I don't feel crushed or anything, just mildly perturbed. Of course it's been compared to The Hunger Games, and I even have a work friend who claims to like it better than THG, which I think is just crazy.
I recently reread Divergent and Insurgent, to refresh my memory, and one thing I noticed was that, unlike, say, Rick Riordan's writing over the course of the various Percy books, Roth's writing never really improved a ton. I never forget, when reading these books, that I'm reading YA, which isn't the case with THG or the Harry Potter books, or other YA I could mention. While there are occasional nice turns of phrase, a lot of her writing feels very childish and repetitive. (The Gathering Place? Merciless Mart? Oy. And I'd kind of love to do a text search for how many times the words factionless and serum were used. Over and over and over... sometimes up to three times in the same paragraph.)
Roth got away with it in the first book, because it was for the most part a coming-of-age story with a dash of romance, but in Insurgent, the narrative turned to what felt like sort of immature political and societal commentary that never connected to anything in the present day. I never felt like I had any allegiance (ha ha) to the "genetically pure" vs "genetically damaged." Allegiant went even farther than that, but that wasn't as irritating to me as the completely half-assed science.
Oh, and pet peeve -- everyone, including the senior officials in the Bureau (another totally creative name, there), seem to be known to everyone else by their first name. That grated on me through the entire book.
I liked the last scene between Tris and Caleb, and her realization that she really was walking him to her execution. But actually killing her off felt... weird. Kind of like when you're reading fanfic and you can tell the author doesn't want to leave things too neat and happy, so Rocks Fall and Half the Characters Die and That's That. It seemed kind of easy-way-out and anti-climactic and... just weird. I suppose if you subscribe to the theory that a story is a promise, the original question of the trilogy was answered: is Tris Abnegation, or Dauntless - because in the end she proved herself both selfless and brave. But it just feels weird. I compare it to the death of another YA character, Rachel from Animorphs. OMG, that destroyed me - I was literally up all night, crying and writing mental letters to K.A. Applegate - but even though it was horrible, it felt true to the message of the series. This just felt... well, I used the word 'weird' already, didn't I?