For a quiet kid who enjoys doing things on their own, it would probably work great. But I could not imagine how my extroverted, funny, bubbly 7 yr old who is brimming with enthusiasm for learning new things would fit there.
I'm currently looking for a preschool for my daughter, and I'm leaning away from Montessori partly because she is introverted rather than extroverted. I know she can work by herself on things for hours at a time. What she's not good at is acting as part of a large group. I want her pushed outside of her comfort zone sometimes. She can mop my floors and clean my shirts at home if she wants. What I don't have access to that preschool does is lots of other kids about her age.
One of my other concerns is that of vocabulary development. From the way Montessori has been explained to me, I don't see how a 3 year old with the vocabulary of a 5 year old would extend her vocabulary much. She wouldn't learn many new words from any of the other students that would be around her. She needs significant verbal communication with adults for that, I think.
I know it sounds odd that I'm simultaneously concerned with what might be her greatest weakness and her greatest strength, but ... well ... I am.
