Sage School is in Foxboro. That's great if you live in the vicinity, but it's difficult to drive to if you live north or west or further to the south of Boston. It's also quite expensive, which may be another strike against it. About three years ago, I spoke to a parent who had a son who went there and is now at Lexington HS; he was on the fence about whether Sage School was worth it or not.
My 2e/pg ds8 attended two of the gifted schools listed on Hoagies -
http://www.hoagiesgifted.org/schools.htm. One was more traditional and structured. The other was not. There were advantages and disadvantages to both of them. It really depends on the child, the level of giftedness, the teachers, the curriculum, the other students, any 2e issues, and on and on. A lot of variables.
All of these gifted schools seem to cater to a certain or specific population and that's great if your child fits their niche, but not so hot if they don't.
We're un/homeschooling ds8 now. There are a number of parents who are un/homeschooling gifted kids because they don't have the money to pay for these gifted schools; and/or they don't live within driving distance to one of them.