This review will contain spoilers!
In many sentences:
Just as I did with the first 2 books in the series, I enjoyed Mockingjay and finished it in just a few days. It was a fast, easy read that was full of excitement. On the whole, though, it was not as good as the first two books for several reasons.
First of all, Katniss seemed like an entirely different character for the majority of this book. Instead of the strong, determined fighter, we get a whiney, medicated little girl. What happened to the Katniss of The Hunger Games? I understand that she went through some emotional trauma, but I had come to expect a character who could deal with her problems head on, rather than whining about it and hiding in a closet. There were a few moments where the old Katniss came through (such as when she fought the Capitol planes in District 8, or when she led the soldiers during the invasion of the Capitol), but these moments were few and far between.
Even more frustrating, however, was her reaction to Gale and Peeta at the end of the book. Are we really supposed to believe that Katniss would be so devastated by the connection of Gale's theoretical development of a bomb that killed her sister that she couldn't ever see him again? But Peeta's multiple attempted murders, and all the history they had together during their times in the Arena wouldn't be an issue? This resolution seemed entirely arbitrary. It gave me the impression that Collins wanted Katniss to end up with Peeta, but didn't have a very good reason for it, so she made Katniss whine about some tenuous connection between Gale and Prim's death. Honestly, I think the idea that Katniss would end up alone, in the woods, surviving on her own as a far more believable outcome.
I'm being harsh in my complaints but, while I think they are entirely valid and unfortunate, it's not as though I hated reading Mockingjay. It was still fast-paced, and full of many excellent moments. My recommendation for reading it may not be as strong as the one I would give to the first book, but this is definitely not a situation like Pullman's His Dark Materials where you should simply stop reading the series entirely after the first book.