Even more captivating than the first two books, Mockingjay was very hard for me to put down. The first hundred pages or so were a bit slow, but once I got into it, WOW. District 13 has started a full-blown rebellion against the capital, and Katniss finds herself LEADING it. This book really just tells the story of Katniss finding herself in the midst of chaos. There's also the ever-present question of who she will choose: Gale, her best friend, or Peeta, who helped her survive the Hunger Games? This action-packed end to the series is just... amazing.
I don't know how she does it, but Suzanne Collins manages to write books that actually get better as the series progresses. Usually, the first book is always the best, but in The Hunger Games series, I'm not sure if that's true. :)
Long story short, Mockingjay is simply amazing. If you were thinking about reading it... my advice to you is to read, read, read! If you haven't read the first two books though, I suggest you do, otherwise things will be very confusing. Other than that, happy reading! :)