Montessori school

Would love to hear people's experiences with Montessori preschool. I'm thinking of sending my 3 yr old to do his preschool years there but worried about how he will transition from there to mainstream school in prep, as most of the kids if not all stay on to finish cycle one (3-6yr old)
Hopefully there's some teachers out there that could give me an idea of how the Montessori kids have gone when Making the switch. There is also the option to keep him at the Montessori till highschool age but then the question still remains the same, how do they then cope in highschool??