Ow - your poor DS Charles! I think generally they teach great values (my two attend one; I am thinking of training to be a Montessori teacher) and the materials help children understand complex ideas, fractions etc.
Some people I know haven't had the satisfaction with them that they have with other settings. If you want big 'results' (e.g. emphasis on school-type activities such as reading/writing) and your child is not pushing to do those kinds of things, they won't encourage or coax an interest out of them. The main ethos is that the 'work' (learning through play, really) is child led - the teachers observe, facilitate but primarily stay out of it and let the child do what they want, as PP said within those simple boundaries. If your child wants to practise writing, do complex maths, try to read etc. they will definitely accommodate that, but as I say it has to come from them. This is in a preschool setting; there may be more structured learning come Reception year or in Montessori schools. I know little about how these work with the 'Method'.
Also, I've noticed since starting school that my DS has become more assertive and confident socially and I wonder if the calmness and general emphasis on activities rather than just pure larking about, such as you may get in a playground, made this side of him slower to emerge.
I think though just be led by your child (no pun intended!) - mine both regularly say/said 'I had a great day at nursery today!' If your DC doesn't give you good feedback then consider another setting.
I think they are invaluable for empowering the child, developing concentration and consideration of others.