My son is at a private Montessori preschool that also has grades 1-6, so I get some chances to hang out with the big kids.
The kids seem great, well accustomed to talking to adults, and intellectually stimulated.
At the same time, it is definitely a hyper nurturing environment that allows much quirkiness to thrive. This is great in some senses, but I feel like some of these kids are either going to have a super tough transition to public middle school, or need to find another hyper nurturing and cloistered option for 7th-12th grades.