The Divergent series is a beautifully written trilogy in which every addition is packed full of adventure and surprises, and Allegiant is no exception. However, despite the fact that there were several more significant events happening in Allegiant than in either of the other two books in this series, I spent my whole time feeling as though nothing was happening at all.
I think the author of this series, Veronica Roth, does her best work when she is focusing on characters rather than plot events; the first book, Divergent, in my opinion, was hands-down the most intriguing and well-written of the three because it focuses only on Tris and the events in her life until close to the end of the book. Allegiant, aside from a short part of the novel, had no character development whatsoever, and focused only on the plot, which was interesting, but it was just not what I was looking for.
Allegiant also featured another viewpoint in its writing- that of Tobias. Although I was excited when I first saw that the chapters were divided this way, I soon realized that Tobias's viewpoint sounded exactly like Tris's... just somewhere else doing something else. The two were so close in writing style that I would get a few paragraphs in and have to recheck the chapter because I couldn't tell whose viewpoint I was reading.
Regardless of my initial disappointment, Allegiant really redeemed itself (in my eyes, at least) at the end, with developments between Tris and Caleb, as well as- less significantly- Tobias and Christina. Their interactions were heartfelt and really left me wanting more, so I was sad with how quickly it ended, but I was overall satisfied with Allegiant.
So in general, Divergent will always be my favorite book in this series, but I'd say every one of them is worth the read.