I finally watched this a few weeks ago. Wow. First of all, great acting all around, especially the kids. Secondly, great story. I hope they didn't embellish this too much, although I know they always do to an extent with true stories. I believe the real-life woman who Naomi Watts played actually had a bit of say in this and made sure that the facts were there.
Bottom line: great movie. Pretty scary, too! You might need some tissues for this.
My grade: A