My son spent his Kindergarten year in a 3-6 class at a public Montessori school in Cincinnati. At first, it was fine, he was doing "well" and liked that he could go from station to station doing his work. About halfway through, during a parent teacher conference, the teacher basically went off on us about my son's "bad behavior", she clearly couldnt handle normal boy energy. It was really bad. My son said that when one child wouldnt stop drawing to come over to group time, she went over and tore his picture in half, then threw it in the garbage. When my son brought cupcakes for his birthday, his father was instructed which kids were not allowed to have one because they hadnt been "good" that day.
I asked for a conference with the principal, and told her (teacher was present)that i didnt think it was helpful to be angry with my son for having extra energy, and then as punishment take away his recess. The principal said that should never be done, the teacher blamed the teacher assistant. It was really terrible. I was ready to homeschool at that point (which had been my original plan all along)but my son wanted to give it another try.
His 6-9 class was better, in that his teacher didnt seem to hate kids, but the asst was like a drill sergeant. This classroom didnt seem at all like the K class, where the kids had contracts and went around to little stations to do work. No, in this class, the kids had workbooks and dittoes. Totally not for my son. It got to the point where i was having to literally drag him into school, he was refusing to go. So i quit my job, moved closer to family, and we are happily unschooling, and have been for over three years.
So thats my Montessori story. Before he enrolled in this one, which was located in a kinda ritzy area, i think it may still have been public, can't really remember. It was located in a beautiful huge old mansion, tons of kids there. I observed for about an hour, and i noticed one child who was having trouble sitting still and cutting out circles or whatever it was they were doing. The teacher was having to constantly redirect him, he seemed really to not want to be doing that activity. Many of the other children were doing their work nice and quietly. There was so much artwork and manipulatives around the room (and yet it seemed the kids were doing lots of ditto type work), and i could not see my son just sitting at a table cutting out circles when there was so much else to look, touch, play with. When speaking with a person from admissions, i point blank told her my son was quite hyper/energetic/spirited and how would they handle that? She immediately brought up medication,and said that she supposed if the parents insisted on not medicating they would have to work with the child on some sort of behavior plan. It really bothered me.
Even though i have philosophical issues with the idea of having to "teach" a child, I do think the Montessori method itself was not such a bad thing for my son. However, i don't think any classroom is really the *best* environment for an active boy. I would caution you to really observe and see which teachers seem to enjoy boisterous boys, and try to get that teacher for your child.