Our experiences with our own children have been okay, but not ideal (now we homeschool).
I will say, though, that the private sectarian school I attended as a child has really taken the modifications my parents talked them into trying for each of us and run with them, to the point that they now have gifted enrichment pull-out 2-6, advanced tracks 7-8, and policies for modified programs, including accelerated courses, independent study, transfer credit from CTY-type or college courses, and partial enrollment for early college (though they won't issue a diploma if you drop below a certain number of core on-campus courses). They also allow students to "challenge" prerequisite courses (test into higher levels by passing the final exam of previous courses). Of course, being a private school, you do pay extra for anything that requires additional staffing.
I had a very good experience there (decades ago), firstly because it was generally a community of warm, dedicated educators who viewed this as a vocation, and secondly because they allowed my parents to assume the risk in adapting the curriculum to my needs. I would probably send my children there if it were not very, very far away. Though I would probably still end up using some alumnal and professional leverage to make further accommodations (especially in elementary).
