I watched it (havent read the book) and was quite suprised that I liked it (cried most of the way through it of course!) What I thought was good was the parallel lives that were being led - one family (the nurse and girl with DS) seemed to have a great happy life with love, and the other one, where they strived for perfection just crumbled... I also liked that they showed the girl with DS doing 'normal' things - college, going on a bus etc and what she couldn't do wasnt an issue.
I also cried all through the film , i found the film much more believable than the book. For me the book lacked stuffing - there seemd gaps in each of the family's lives . Whereas in the film as you say -you saw both families - lovely parallell that having a child with DS actually enhanced one family whilst the other seemed to lack in some way. As you say it was great to see the girl just being a girl, I loved it when the girl & boyfriend were on the sofa snogging & then announced they were geting married ! Awful to think that not so long ago (relatively ) our children were not entitled to an education, just as it was for them in the US