J. Michael Straczynski's script (one where every plot point can be verified, which almost never happens with based-on-reality movies like this one) is, ultimately, a simpler story than it acts like, if that makes any sense; the film got shot based on Straczynski's first draft, both because the producers felt it was already a solid script and because it meant the film could get into production before the 2007 writer's strike, and it feels like a first draft. I wish Straczynski and director Clint Eastwood had had the chance to rewrite it: still stay as true to the events as the final film was, but making it a more well-rounded piece of art. It gets most of the way there, but not quite.
And the performance of the man who lives at the farm (I'm trying to avoid spoilers here) is, to me, just on the wrong side of just a little too over-the-top. His performance, for me, doesn't quite work. Meanwhile, I was impressed with as bold a performer as Angelina Jolie portraying such a contained, usually quiet person as Christine Collins.
Still, not a bad film, and good at creating a sense of place when recreating 1920s and 1930s Southern California, down to the somewhat unexpected accents. I'm a sucker for interesting accents.
Want a more detailed ambivalent reaction to Changeling? Read Mike Russell's review.