I have heard that Montessori are great for preschool & K, but transition to regular classroom could be difficult.
People love to use that line.
The only trouble that we had transitioning either of my two girls were:
1) They were ahead of the class they entered. Way ahead, but since Montessori is about YOUR child, not how they stack up against others, I was blissfully unaware just how far ahead they were.
2) Their expectations of their teachers was too high. The new schools just couldn't match the deep respect Montessori teachers have for their students.