Depending on who you ask, "Mockingjay" is either the worst book of the series or absolutely amazing. I tend to fall in line with the latter.
And, please be warned, there have been spoilers lurking these paragraphs ahead.
After being dramatically rescued from the nasty Quarter Quell edition of the Hunger Games, Katniss finds herself part of the rebellion against the Capitol. Unfortunately, in the chaos of the rescue, her sometime-lover Peeta was left behind and is now a prisoner of the Capitol. Katniss struggles to find her place as part of the rebellion: she is important as a symbol and a figurehead, but President Coin of the rebellion is wary of giving Katniss too much power. As the rebellion begins its final assault on the Capitol, she struggles to keep herself and her family safe while also leading the way to a better, brighter future for all.
I think most of the criticism of this book stems from the fact that this book is, by far, the most cerebral of the novels in the series. This time, we spend a significant amount of inside Katniss's mind, in place of the action which dominated the earlier books. Katniss is a much weaker character in this book; she has been broken and beaten down too many times. Rather than driving the action of the plot forward, she is much more of a passive observer (especially in the early part of the book). She spends much more time thinking "deep thoughts" about the nature of abstractions like power and violence, good and evil (and, yes, a few moments thinking about Gale/Peeta dilemma). It is not until the final few scenes that the strong-willed Katniss returns.
This departure from the standard established in the first two books is a turn-off point for many readers, who were hungry (pun intended) for more of what they got before. I actually appreciated this change, though. It always felt like there were bigger philosophical issues hidden underneath the plot, but that those issues were always neglected in favor of keeping the story moving forward. But, if Katniss were as intelligent as she showed herself to be, these things must have crossed her mind at some point, and, finally, in this book, we get the chance to get her opinion.
I appreciate the fact that, when the assault on the Capitol begins, the action of this book is strikingly different than the last two. Whereas the first two books took place in the Hunger Games arenas and the violence was mostly limited to participants, the violence in this book takes place in cities and pretty much everyone is a fair target. (And, trust me, there are a few surprises as to who gets hurt and / or killed in this book.)
I will say that I was slgihtly disappointed in the "Epilogue." Frankly, I didn't need it. I've voiced my opinion about these types of "gift wrap" endings before to anyone who would listen. I can't stand it when the author (or screenwriter, or director, or whomever) decides that the audience needs everything spelled out and resolved for them. Granted, I appreciate that not everything worked out perfectly for the characters: Katniss and Peeta are both ruined human beings, and several key figures are dead. So, I don't hate this ending as much as I usually hate endings like this, but I'm not terribly happy with it either.
Overall, the entire "Hunger Games" trilogy was a decent read, and I'd generally recommend it. Anyone going into it should realize that the series is not "classic literature." That being said, the books are fun to read, and well written enough to hook readers in with lots of twists and cliffhangers.