I hardly post here but would like to share our experience. Sorry if I am duplicating any info as I am yet to go through previous posts.
DS4 has been going to montessori since last year. This year the school started on a rough note due to a teacher change hence new rules but given a month DS is adjusting well. In his first year he chose a lot of language and math work but this year he is not interested in them. He is spending his time in art, science and geography corner and seems happy with a few rough patches once in a while which we try to deal with as they come. He is definitely learning, not in math and language but in other areas. He is experimenting with different strokes in art, learning about weather, plant and human life cycles, people around the world, geometry etc. His school also offers a third language, lots of field trips, fun projects and computers.
At times we do feel that we are paying a lot that too for something, I could easily teach him but he loves the social aspect too, plays with older kids as well as his peers , ah well most of his peers are 5-6 year old too but most days he comes home being very proud of what he did at school and will introduce me to new kids as I go to pick him up. We are keeping our fingers crossed, hopefully each year he'll have something new to look forward to.

Montessori is not for every child though. A lot depends on the child's personality, drive to learn and how much\lesss guidance a child prefers. I have many friends who pulled their kids out of the school as they didn't like it. One friend as she put it, did not like it as all her child would do is to play with puzzles. DS's school however has a policy that each child has to choose at least one challenging work each day.

A montessori classroom is very quiet during work-time, the schools usually have a zero tolerance policy for any disruption, hence strict rules. Go visit a class and see if you can imagine your child doing well there. For us it is working well so far if not great. But we are skeptical how long its going to work but we will see:)