During my last year in NYC, I used to walk the dogs with a group, including a father (princeton grad), whose daughter has a unattractive SAT score. She did the prep and significantly improved her math score. Also, don't know if it was prep or what, but a daughter of a friend, who also just graduated from DD's school, had a bad SAT and retook and had a significant change. Got into Stanford early. So not first hand experience. I know the first had prep involved. But both had significant changes in SAT scores.