This is the sequel to The Hunger Games, so it's a bit limiting what I can write without being much of a spoilter. The first book, while self-contained wasn't really satisfying - the protagonist survives, but the setting is so oppressive that survival within that framework is a minor victory. So, making the book into a trilogy doesn't feel that forced.
This book develops the backdrop world somewhat further. On one side, this is not a great idea, as it gets somewhat economically implausible, and the totalitarian politics are still alien. On the other hand, it ups the tension and scale, and given the society, the actions of the individual people within it seem to work. In terms of developing from the first book, it's successful.
Katniss is less irritating in this book, perhaps a sign of the character growing up, but the author can get away with a lot from having a teenage main character.
Towards the end, there is a twist, but it's somewhat undermined by the fact that actions that seemed logical before actually make less sense once the twist is revealed. I can't really say more than that without throwing in further spoilers.
A friend of mine viewed the second and third books as a cash-in on the successful first. I think this is unreasonable - the quality is on par with the first, and it develops the plot line well. Fingers crossed that the third book rounds it all out nicely.