I'm going to try to do it because there are plenty of other websites where you can go to get spoilers, so the most I'll say is the ending is a bit of an unexpected shock and readers have been divided over it. But for me, the book was a bit of a let-down in other ways and I think it was almost the ending that "saved" it. The plot as a whole kind of meandered around between two fairly indistinguishable points of view (Tris' and Tobias') for a long time and then climaxed quickly and shockingly in the last few chapters. Without that ending, there wouldn't have been much memorable about the book.
Now, that does sound pretty harsh and I really don't mean it to. Veronica Roth is an amazing writer, particularly for someone so young. And I think the real problem she created for herself was at the end of book 2 (Insurgent) where she changed the rules of the game (ie the dystopian world) entirely. It was kind of like "you know everything you readers have invested into this society over the last two books, well it was all made up". Having done that, and having turned all her wonderful world-building work into a bit of a paper tiger, she had limited options for where to go in book 3. So I guess I didn't have particularly high hopes for it, and given the difficult task she had set up for herself at the end of book 2, she really didn't do all that badly with the material she had left to work with. Her writing style is still engaging and the characters are still relatable (although they are all a little "flatter" and more "two dimensional" in this book than the previous installments).
In some ways the other problem Roth had created for herself at the end of book 2 was that she had really completed her heroine (Tris') character/story arc in that book and she had to make book 3 more about developing Tobias' character, but he's in many ways simply not as interesting a character as Tris. He was also a less "likeable" character in book 2 than book 1 so, as a reader, I was a little less willing to become engaged in what effectively turns into his story in book 3.
Having said all this, I've also read that Veronica Roth has been rather dismayed about the reception of this book and I can't help feeling bad about that because the trilogy as a whole is a masterful achievement even though the third book kind of goes off on a tangent and loses some of the pace and energy of the earlier installments. So, unlike some other reviewers, I wouldn't say that the third book spoils the whole trilogy for me or that I'll never read it again. And I will definitely read more of what Roth writes in the future. I simply think she unwittingly set herself up for a fall in this final book by where she decided to take us all at the end of the previous book. So Be Brave, Veronica! You're a great writer and I hope to see lots more from you in the future.