I did Montessori for preschool and kindergarten, and my review of it is that it will definitely trampoline your kids knowledge in certain subjects they have interest in relative to other kids their age who go to a normal school. I was very strong in math going into 1st grade, but was practically illiterate because I didn't like reading very much.

Your kids will probably be able to socialize with people outside their age range better, but will not be as socially adept with kids their age. I'd recommend some outside activities with strictly people in their age group.