I think she made the right choice.  Not speaking of the movie, because I don't much care (though I agree with lentzk about the casting), I think the reason Katniss chose Peeta is because she did not like Gale's values.  Gale made some decisions during the war that she did not approve of.  She did not agree with his weapons, and what were basically war crimes on innocent civilians.  She had also been through a lot with Peeta by that time.  No one else could understand her like Peeta.

THANK YOU #5.  I thought Peeta was horribly cast.  There is no way he should be shorter that Katniss, and he comes across much too arrogant.  In my mind as I was reading the books I imagined Peeta as a 6'2" version of Sam from the Lord of the Rings film.

As for Gale, I think they cast him as too "pretty".  He was a poor kid, scarred and rough.  I didn't picture him as muscular, but fit.  The guy in the movie is WAY too healthy looking to be Gale, who probably gave his little brothers and sisters more food than they deserved while sacrificing himself.  I guess they had to make sure all the teenage girls had a crush on the characters though.

I have to admit, I was pulling for Peeta all along, but I would have been okay with her ending up with Gale up until the 3rd book.  I think Gale's personality in the 3rd book was telling of the different directions he and Katniss would ultimately go.  Add to that, the impossibility of anyone to understand her as a surviving tribute, except another tribute, and Peeta was the likely choice.

(I have to admit, in the film, Gale is a much better casting job, in my humble opinion, than Peeta, and I'm thoroughly disappointed that they selected an actor who was both short and not naturally blond for Peeta.)

I don't think the love story was really a key part to the story.  It was really in the background of the story rather than a main point.  We could say that Gale is the direction Katniss's life may have taken if she had not entered the arena.  Of course, she wouldn't likely have married anyone if the revolution had not occurred.  Gale and Katniss were too much alike.  Katniss changed so much that she and Peeta were much better suited for one another.  Gale could never understand her the way Peeta could because he did not experience the same things she did.  I think it was the right end to the story, but the love story really wasn't the focus.  Her marriage to Peeta instead of Gale shows that the characters have changed.

I was really indifferent about who she would choose, and really wondered if she would end up with either of them to be honest.  There was a part of me that thought she and Gale were a better match because of their history together in the Seam, but another part of me that thought Peeta was a better fit because of the history in the arena.

In the end, I think Collins made the right call... but I wouldn't have been mad if Gale had been the one Kat ended up with.