Often in a trilogy, the middle book is a somewhat less interesting bridge between two more interesting and engaging books. Technically true in this case, as there's a whole lot going on in Mockingjay. Hands down the best part - Cinna's costumes. Worst part - everything else. The plot twists were painful, there was too much gore for what's supposed to be a YA novel, and the direction the love triangle took nearly made me ill. (Oh, and don't get me started on the resolution.)
I find that YA books tend to fall down when they model adults, and especially large groups of adults (institutions, governments). They end up individually acting like children and collectively like catchphrases. And it happened here. Witness Russia District 13 and Rome the Capitol, who seem to mostly be driven by the whims of their leaders.
Katniss continues to alienate the readers with her selfishness (exception:

Spoiler

her empathy for and defense of the hospital in... I forget. 8?

) and general moroseness, with a few turns near the end really driving the antipathy home (

Spoiler

voting for a last Hunger Game (while I'm on that, what's with that? Coin's council couldn't decide what to do about the citizens of the Capitol

(and while I'm on that, why "Capitol"? - a capitol is a building, and a word that too many people already think means a city, so why try to confuse a generation of readers even more?

Spoiler

, so she got some celebrities to decide), and her decision to suicide

).

Anyhow, the book was basically a long series of more or less uncomfortable scenes, to the point where I was 17 pages from the end and seriously considering not finishing. If somehow you're reading this without having yet read [b:Catching Fire|6148028|Catching Fire (The Hunger Games, #2)|Suzanne Collins|http://photo.goodreads.com/books/1268805322s/6148028.jpg|6171458], save yourself. Stop after (or even before) [b:The Hunger Games|2767052|The Hunger Games (The Hunger Games, #1)|Suzanne Collins|http://photo.goodreads.com/books/1326003698s/2767052.jpg|2792775]. Go read [b:Among Others|8706185|Among Others|Jo Walton|http://photo.goodreads.com/books/1317792367s/8706185.jpg|6449955], [b:Plain Kate|7877239|Plain Kate|Erin Bow|http://photo.goodreads.com/books/1328772028s/7877239.jpg|7160746], or [b:Tender Morsels|2662169|Tender Morsels|Margo Lanagan|http://photo.goodreads.com/books/1320416424s/2662169.jpg|2687395] instead.