It is about as good as any other multi-year commitment, leadership/outdoors/civic involvement activity with nationally recognized standards. The BSA largely tries to inspire boys to be service minded, well-rounded, fit, patriotic, and to gain leadership experience. That lines up fairly well with what the service academies are looking for.
So, I'd call it one of the stronger extracurricular activities out there, from an admissions standpoint. Of course, it's only one thing, so don't think that they'll be wowed and just hand out an appointment based on it.
When I applied, some years ago, I recall the application having check boxes for things like varsity sports letters, team captain, CAP Mitchell Award, and Eagle Scout.