It could be great (it is a relatively challenging sort of curriculum) or it could be awful... like any setting, it depends on how it's actually executed.
The one difficulty I can think of off the top of my head is if they interpret the stages very strictly by age. I know among classical homeschoolers you occasionally get one who says "you are X years old and therefore you are in the Y stage. period." That's right up there with "12 year olds aren't developmentally ready for algebra. period." My issue isn't with the original statement (which is likely true for a large proportion of the population), but with the inflexibility.
If they're willing to adapt to the individual students a bit then I think it could be a great curriculum for gifted kids... definitely worth looking into!
