The good: Plot twists and out-of-nowhere deaths everywhere keep you guessing. Collins does an impressive amount of physical and psychological damage to Katniss, far more than in The Hunger Games or Catching Fire. The change in her character over the course of the series is amazing. There's a war between the districts and the Capitol, of course, which brings plenty of fear and tension to the party. We finally get closure on the Katniss-Gale-Peeta love triangle without it consuming the plot, and we finally get some characterization of Prim beyond "Katniss's cute sister that she'd give her life for." Actually, many minor characters are more deeply developed. Collins isn't afraid to introduce huge, character-changing plot twists, isn't afraid to completely destroy a character's life and then gracefully follow the effects.
The bad: Katniss spends most of the book as a figurehead for the anti-Capitol rebellion, which keeps her fairly passive. Life in District 13 is also very structured, and her only rule-breaking for a long stretch of time involves finding custodial closets to nap in. Instead of becoming stronger and turning situations to her advantage as she did in the previous two books, Katniss is torn apart physically, mentally, and emotionally, but never really put back together. The resolution of the love triangle is also a bit lackluster, though I was satisfied with the outcome. Overall, the book lacks the tremendous energy and fighting spirit of The Hunger Games, and so does Katniss. I feel that Collins tried to do too much, and perhaps another 100 pages or a more character-driven plot could have saved the day.
Overall: You have to read it if you read the first two, which you have to read or you're just missing out. While the book contains a huge dose of pain, it's a product of the larger story. It ends the only way it really can. This story was not a happy one, and a truly happy ending would have been a lie.