I agree that it did well to set my up academically. I ended up going to middle/high schools that were relatively average academically, so I was pretty strong in all subjects in comparison to my peers. Ultimately I went to a very prestigious college and now work in a wonderful finance job I love. However in the many years of therapy I’ve had as an adult, I continue to identify Montessori school as a foundational contributor to my social anxiety, and ultimately my ensuing clinical depression over a lack of social life which haunted most of my college years.
My Montessori school had about 15-25 kids / class year. Some of the larger years were split into two groups with separate teachers. Every year, maybe one or two kids left to go to other schools and one or two new kids joined, but for the most part I grew up with the same core set of children for seven years. I honestly believe this had a permanent negative effect on my ability to socialize and form new friendships that I am only barely beginning to correct over a decade later. Admittedly I did not participate in any extracurricular outside of Montessori school (particularly because it had its own after-school programs). So when I transitioned to a public school for middle/high school, it was a sharp culture shock and I definitely struggled to fit in.
I think Montessori schools are worthwhile academically, but you should be careful to keep your children in contact with other kids outside the Montessori bubble.