I really, really like it.
I think it reinforces the kids natural curiosity.
In middle school, my first year out of Montessori, I was shocked at how little other kids cared about learning. I remember the teacher discussing something about astronomy, and I raised my hand to comment on some fact I had read, and what followed was mockery by my peers and antipathy by the teacher. I learned quickly to never again show that I cared about learning.
This was a huge contrast with Montessori where most us were eager to learn and share what we had learned. I had friends that had built the solar system to scale out of their own initiative (in hindsight they may have taken some liberties, nonetheless).
I kept tabs more or less my classmates that came out of the Montessori, and I think they overall overperformed the non Montessori people in middle school and high school. Harder to gauge adulthood success.
I also liked that they had children of various years in the same classroom. I think it promoted knowledge sharing from the older kids to the younger ones, and it removed barriers for friendships. Some of my best friends back then where older than I was. That would never happen in middle or high school.
Finally, I don't think it's perfect. Because we were all expected to join a traditional school after grade 6, the school made some effort to make sure that the outgoing class had covered all the basic requirements (a not necessarily a simple thing since we had great liberty of pursuing what was interesting to us).
All in all, I would strongly recommend it.