Montessori for 5 years, myself. I mostly learned how to learn, and the struggles later were that my education was well beyond that of my public school peers when I switched. I found learning by rote to be a waste of time, and continued to learn in my own ways.

As a result, I had amazing test scores in public school, but bad grades overall because I felt that the homework was a waste of my time.

I still appreciate my time on a Montessori school though.