It's interesting to see how many different reactions there are!
I appreciated the violence and brutality--I actually felt there wasn't enough of that in the first two books (one of my major peeves with HG is that Katniss never actually has to kill someone in a way that isn't sort of an accident or out of mercy), and I wish there'd been more of it at least in CF so that the tone shifted more naturally. I can see how people would be jarred by the change. I liked that Collins didn't shy away from just how awful war is, or what effect it has on people.
Also really enjoyed some of the twists, like Peeta's hijacking. And I didn't see Katniss as passive--many times she breaks orders and does things because *she* thinks she should do them, even though the people trying to control her would rather she didn't. Yes, she isn't in total control, but is anyone else either?
What disappointed me, though, is a lot of the emotional stuff felt off. There isn't a whole lot of time for emotions in the first two books--everyone's running around preparing for games and then trying not to be killed most of the time--but in this one Katniss spends a whole lot of time reflecting on things. Which could have been good in theory, but I didn't like the way it was handled. For one, it felt like Collins often pulled away from moments that should have been emotionally intense right as they started to happen, and then came back to Katniss hours or even days later when she'd reflect back. The first time it really struck me was when Peeta attacks her, and it also bothered me when Prim dies--I didn't even know for sure Prim *had* died based on what is on the page in that moment. I wanted to see Katniss reacting to things in the moment, rather than just after she'd already had some time to process them.
I also didn't like that we're almost completely left out of Katniss's thoughts and feelings when she's hearing about Coin's proposed Hunger Games, deciding to shoot her, and then finding out what happened after. She gets *very* distant at the end. Honestly, the way it's written, I'm not sure we're even supposed to believe Katniss really loves Peeta. As much as I do adore the last "Real/not real?" exchange, in the lines before she's not talking about feeling anything for him... She's talking about what she "needs" and what Peeta can "give" her, and because of that, she tells him it's real that she loves him. She never says it's because it really is real.
Which would be fine. It's just that when I've stuck with a character through three books, by the end I want to feel closer to that character than I have before, and instead I felt more distant. I wanted some sense of how Katniss felt about the decisions she'd made over the course of those books and how things had turned out for her, but she almost didn't seem to care about anything (not about her kids, which she admits she only had because Peeta pushed for them; not about how her society is changing or the role she played in making sure her kids won't have to experience Hunger Games). Which maybe would happen to someone who's been through as much as she has. But it makes for a pretty depressing ending, if the main character can't show even a little happiness out of having made it to the end of that journey. (I just re-read the epilogue to confirm my impressions, and I still feel the same way--the closest thing to happiness is when she mentions "joy" when she held her daughter for the first time, but I don't *feel* any joy actually shown in the way it's written.)
So... It's a book that's stuck with me after reading it, and that affected me a lot while I was reading it. But in the end I feel kind of unsatisfied.