I hated the ending to Mockingjay. Catching Fire was meh, but it was at least able to capitalize on the momentum of the first book. Mockingjay -- particularly the ending of it -- was just terrible. Unlike the other folks here, I'm ok with Katniss leading the suicide mission into the city. Where things go FUBAR for me is when they leave the shop and:
1. Prim gets killed. Why was Prim even there? What purpose did it serve either in terms of story or plot? Was there any purpose in Collins putting her there other than to give a big F.U. to Katniss and the readers?
2. Katniss devolves further into a raving mute lunatic, until one day she just decides to wander into Snow's rose room. Snow is all "well, are you sure I'm the real bad guy?" and Katniss just kinda goes "oh, alright then. That's kinda what Boggs was telling me, wasn't it...", and you know she's going to kill Coin, even though there's very little evidence that Coin is really some kind of horrible dictator so much as just authoritarian.
3. Katniss votes for the final Hunger Games. I really don't buy the contextual arguments some of you are making for her having an excuse to shoot Coin. Katniss was downright unstable at that point, and I seriously doubt she was thinking that in-depth about it.
4. Her trial goes on without her, without her finding out anything about it, until she's apparently acquitted on temporary insanity.
5. She goes back to District 12 and ekes out an existence, until Peeta comes and lives with her and they live somewhat happily ever after. She never sees Gale again, or her mother, or anyone else besides Haymitch. So the world's a POS, and so is her life, but somehow we're supposed to be convinced that this is still a happy ending.
I have seriously never been more disappointed with a work of art in any medium. The book just takes a total nosedive in the last 30-50 pages. I had to reread the chapter where she wakes up in the hospital to make sure I wasn't being deceived into reading a dream sequence. It's as though Collins went out of her way to disillusion her entire fanbase and ensure nobody ever reads anything of her's again.