Our son attended a private Montessori school beginning when he was 18 months old and on until he was in the equivalent of 1st grade.
The school was established by a Montessori teacher who saw a need in her local community for better educational choices for children who come from low-income families. She established the school in one of the poorest neighborhoods in Dallas, and you had to meet certain income requirements in order for your student to attend (you have to be below poverty level). The school followed the Montessori method.
At the time, our family was very poor and the public schools were horrible in the area where we lived. We also could not afford preschool at the time. When we heard about this school - we quickly got on the waiting list - and it was a long list! Luckily we got on the list while I was still pregnant.
Anyway - our son had a wonderful experience there until he graduated the Infant/Toddler program and transitioned into the primary program. When he was about 5, the two Montessori teachers in the primary classroom said he wasn't "self-motivating" himself enough, and that he would sit in one area of the classroom and look at books or play with cars the whole time he was at school, and at recess all he wanted to do was run around and play on the playground.
I thought to myself, really? A 5 year-old kid that wants to look at books and play with toys and run around all day and this is a concern for you?
Anyway... the school required parents to volunteer and so I was there at the school quite a bit. I had always kept an eye on what my son was doing in his classroom, but I began keeping a more critical and watchful eye on the teachers.
They seemed to be more like babysitters than teachers. No real guiding, no lessons, no ... teaching. They basically just maintained peace and quiet, and showed kids how to use the blocks and materials and "stations" that were set about in the classrooms.
Anyway... our son had a great experience in the Infant/Toddler classroom, but beyond that he needed something more structured. When we took him out of the Montessori school - at the age of 6 - he still wasn't reading or writing. I taught him that over the next year while we homeschooled and worked towards moving to a better neighborhood with better schools. (We ended up moving out of state.) He learned more while we homeschooled than he learned the year and a half he spent in the primary classroom at the Montessori school. We did writing, math, reading, and science - he wasn't getting any of that at the Montessori school. Not in the traditional way... it was just sort of out there if he wanted to pick it up - but what kid wants to learn his letters or do the hard work required to learn how to read when there are cars and blocks to play with and cool books to look at and nobody is going to make you do anything else?
Maybe it works for some kids, but for our son he definitely needed a program with a little more push...