I feel like this is one of the main questions that drives Mockingjay. Yes, there are about a billion other things going on at the start of the novel, but this is one of the main conflicts that Katniss has to deal with. Never fear, this isn't a spoiler. I think it's quite clear in Catching Fire that it will come down to this, I might even go as far as to say that it's clear in The Hunger Games. You know from the get-go that Katniss and Gale have a special connection, and Peeta and Katniss obviously form a special bond over the next two books. But who is she going to choose? I feel that readers were torn in this, kinda like readers of Twilight were torn between Edward and Jacob. But instead of having two really poor choices (let's face it, Edward and Jacob don't make the greatest boyfriends...), Katniss has two really amazing guys that she feels strongly about, though in different ways. Personally, I wanted her to be with Peeta because he's much sweeter than Gale, and I felt that if I had to choose between the two, I would definitely choose Peeta. But many people disagree. Who would you choose?
Obviously, there is a lot more going on in this novel than just Katniss's romantic choices. A war is going on, and Katniss is in the center of it all. Though she's trying to escape the clutches of the Capitol, she finds that the Rebels are almost as bad as the Capitol, still always telling her what to do. Katniss really struggles with her position in the war throughout the novel, and at times, it wears a little on the reader. You kind of just wish she would do something, though you do realize that she's kind of stuck between a rock and a hard place. A lot of people could get hurt no matter what she does.
Overall, I think that this was a pretty good ending to a trilogy. It doesn't leave you wondering what happens to any characters, Suzanne Collins made sure to show what happened to everyone you could possibly care about. Always good when there's no cliffhangers. And despite how sad and depressing the whole series it, it kind of has a happy ending. Not completely, but mostly. It's always good when there's a least a little happiness in the ending. Makes the reader feel a little bit better about life.