The more I think about this book, the less I like it. I started with 4 stars, then went to 3, now it's 2. I first thought it was a good book with a depressing message; now I realize it is a bad book with a condescending, obvious (and poorly delivered) message. 
 There are several problems with the writing: 
 * I realize that it's realistic that Katniss would break down after all she's been through, but she spends half the book catatonic. It not only makes for very boring reading, it also completely destroys the character we grew to love after the first two books, and it is much more dark and twisted than anything that preceded it. 
 * Even more importantly, it makes the book go incredibly slowly. This leaves her with very little time so Collins has to rush through the things that actually matter and treat the deaths without any of the care and attention she gave to (for example) Rue's death in book 1. People drop like flies, and neither the other characters nor the reader have time to give a crap about it. 
 * Finally, it also removes from Katniss' final breakdown at the end (ultimately, the only one that matters) any sense of pathos. At that point, the reader is just "what, this nonsense again?" 
 * Too much of the action takes place "off screen" with Katniss in sedation or in a hospital room. Once or twice would be ok. But Collins returned to this trick again and again until it was just boring, an excuse for wrapping up complicated plot points in a few sentences instead of working to write the scenes properly. 
 * The gratuitous gore which Collins avoided in the first two books shows up with a vengeance. At one point Collins even directly refers to the evisceration of children and the scattering of their body parts. 
 * The technology is unrealistic. The behavior and motivation of most of the characters is unrealistic. The action is unrealistic. 
 * The concept of the Capitol being a Hunger Games arena was poorly executed. The handling of the fake promotional war footage was poorly executed. 
 * There are several grammatical and typographical errors. Nothing major, but enough to provide additional evidence that the book was rushed. 
 (view spoiler)[As one example of the poor plotting, President Snow has just gone through a lot of trouble to create a human shield. Why would anybody believe that immediately afterwards he blew up his own human shield? Yet, Katniss only gets away with killing Alma Coin by virtue of being insane, rather than the more satisfying mechanism of actually proving that Coin was the one who blew up the human shield. In general, a good author doesn't spell out everything for the reader. But in this case, it was imperative for Katniss to resolve this, which would not only have been more generous to the character but would actually have been more realistic. Instead Collins takes the lazy route which simultaneously is poorer story-telling and inflicts yet another unbearable psychological blow on her own heroine. (hide spoiler)] 
 The last few chapters are probably the most disturbing ending I've ever read. The 2 + 2 = 5 ending of 1984 and the world blowing up in A Canticle for Leibowitz are of course extremely dark, but they make important points. In Mockingjay, the darkness of the ending is utterly pointless. Not pointless in the way Collins intended (war is pointless), but rather in "why did I just waste hours of my life reading this series when the author has nothing useful to say about the human condition?" I didn't want or expect a happily-ever-after ending, but I did expect a satisfying ending. I most definitely didn't get it. 
 (view spoiler)[I can buy the torture of Peeta. I can also buy Katniss giving up on him because of it. I can buy the death of Finnick. I can buy the destruction of Gale's soul to the point where he thinks that the two-layer bomb to specifically kill medical responders is a great idea. I can even buy (to an extent) that Katniss became broken and empty. But to do ALL of them is nothing but an exercise in sadism towards both the characters and the readers. Even what Collins calls "happy" events (her own word in the final chapter of the book) is nothing but more darkness and depression. Annie and Finnick had a baby? Its (we're not told the gender) father is dead, and its mother is insane. Really happy. Thanks. Nice touch. 
 Most appalling of all is the unexpected and meaningless death of Prim. Defenders of the book can call this a realistic depiction of war all they want, but it's nonsense. It wasn't foreshadowed or set up, so when it happens you literally have to go back and read the sentence again to make sure you didn't misunderstand. There's no suspense, and no time to think about what it means, since you didn't even know Prim was going to be there. And Katniss ends up in a hospital again, so there's no time to mourn her afterwards either. The whole thing is just one more sadistic attack on the reader. There's ZERO motivation for Coin to kill her. With the two-step bomb, there's at least the (implausible) rationale that she can frame the Capitol and end the war faster. What possible benefit does Coin get out of murdering Prim? Breaking Katniss? We can see how that backfired (in a way that could easily have been anticipated). The fact is killing Prim gave Coin absolutely zero benefit and exposed her to considerable risk, particularly losing her legitimacy before even taking power. I've read books where authors have violated logic to save a character, but violating the premise of your book to KILL a beloved character is just an abuse of the reader's suspension of disbelief. The plot device of Coin killing Prim is so monumentally stupid and implausible that I've actually just gone back and taken a star off my rating of the book. 
 As another (less important) point, the handling of the Peeta reunion was rather poor. Of course she couldn't end up with Gale after he designed the bomb that killed Prim, but Collins could have handled ending up with Peeta much better (particularly after her treatment of him throughout the book was utterly atrocious and without any redeeming value). The book actually would have been slightly less morose without the epilogue, since there was at least a glimmer of hope in the "Real" at the end of the last chapter. (hide spoiler)] 
 The entire book is one huge bait-and-switch where Collins brings you in with the first two books and then uses the third book to beat you over the head with her condescending patronizing over-the-top war-is-terrible message at the expense of destroying her characters and her storyline. 
 There's far too much description of the horror of war. If I want to read about the various ways that ordnance can mutilate the human body I'll read Turtledove's WW2 series. Where's the interaction between the main characters? It's non-existent. What's that you say, Suzanne? War is violent and pointless? Thanks, I knew that already. The book was depressing. What's that? War is depressing? I know that; you just said it; you can stop now. 
 It's odd that the book I was reading immediately before I picked up The Hunger Games was Studies in Pessimism by Schopenhauer (widely acknowledged as a depressing philosopher, whose book The World as Will and Idea has actually caused suicides), and the book I'm reading now is The History of the French Revolution (ending with the Reign of Terror), and both of those books are actually considerably less depressing that Mockingjay. Collins actually succeeded in making her book MORE DEPRESSING than real life. That's quite a feat. 
 Something really terrible must have happened to Collins to make her want to inflict this extreme torture on her readers. (I know her husband is a military strategy expert; did she lose a friend in combat between book 2 and book 3?) The fact that this book is targeted to young adults only makes the unremittingly disturbing tone that much more offensive. She should be ashamed of herself, and I'm no expert but she should probably seek psychological help. 
 I apologize that this review is not particularly cogent, but I just wanted to put some thoughts down as catharsis to try to put this book out of my mind. I'll probably edit it later to improve coherence.