I went to a Montessori school between kindergarten and 2nd grade, and I believe it served me well in comparison to what I would have gotten out of public schools. In NY state public schools in the U.S., at least the Montessori education I received between those grades left me about a year ahead of where public schools were in 3rd grade. This is purely anecdotal of course, and vague memories at that. It may be that there is a good deal of variability between the quality of different Montessori schools, or that my experience was the exception to the rule, but I wouldn't write off Montessori schools so quickly.