Honestly, I'm satisfied with the conclusion of this series. I'm not angry, I'm not heartbroken, or any other strong emotion like that. I'm a little sad for Tobias, mostly, but that's it. I thought this was better than Insurgent at times, but, even though I read it in a day, it still felt like it dragged at times. That could just be me, though, because I feel awful today, idk. Anyway, I think there was more filler in the second and third book than was necessary, and this probably honestly should have been a duology, or at least a trilogy with a shorter second and third book.
As for Tris's death, I wasn't surprised at all by it. I think it felt more honest and true to who she was than any alternatives would have been. I kept expecting the end to reveal that she was just the subject of experiments in Erudite--with Jeanine still working to find serums that the Divergents couldn't fight--or something like that. Then at the end, when Tobias was considering drinking the memory serum stuff, I was worried he was going to do that and forget, and then it would be revealed that Tris's death had been faked to protect her, kind of Romeo & Juliet-ish. I'm really, really glad none of those things, or the others I imagined, turned out to be true. Maybe if I'd loved Tris I would be more upset by her death? IDK. I just know that I liked her alright, but I wasn't too attached to her.