My children also took either the SAT (DD) or ACT (DS) in 7th grade through the Duke program. DS took the ACT last week, so I don't know how he did yet. He stated that almost all of the math section and most of the science section involved material he hasn't covered yet, so he probably didn't do well. If the practice test he took at home is any indication, he did well in the verbal section, as did DD when she took the SAT.
I agree with others who felt it was beneficial as my children were able to take the tests as a lark, with no expectations or pressure to do well.
DD did well on the PSAT, SAT, and ACT, so only took each test once. Some of her classmates took them multiple times to a. improve a less than stellar score or b. trying for a perfect score.