If you live in McLean, you are greatly simplifying your quality of life by living near your school.
Potomac is a very good school, and frankly, not having to commute through daily traffic allows your children more time to enjoy school extracurricular activities and athletics, and still get home easily to start their homework. You also have the additional advantage of having many of their friends within easy driving distance, in that way it is like a neighborhood private school.
I think that it is a very legitimate decision to choose Potomac over one of the Big 3 schools, and particularly for Primary School. No, I do not send my children to any of these schools.