I have a friend who had the test and, based on high risk results, opted for the follow up amnio. They had a few weeks of worry and stress and in the end the baby was perfect. Their experience just reinforced for me the decision I had made not to have the scan.
Having said that, I know someone else whose 12 week scan showed severe abnormalities that would have resulted in stillbirth or late miscarriage. It was easier for them to come to terms with losing the baby early in the pregnancy rather than later.
It's such a personal decision and it has been good to read this thread to see some different perspectives.