We have had great success with Montessori. Our son (now six) had a miserable experience with our state-run pre-k. We took him out and put him in a new Montessori school.
He was there last year (which would have been pre-k) and this year (which would've been kindergarten). Our school identified issues which lead to evaluation and occupational therapy. He is simply a different kid. Much happier, more social, more physically capable.
Montessori isn't always a perfect fit, however. Our son is not a self-starter, which is a main thrust of Montessori. The teachers, however, would allow our son to initiate. After a while of watching him choose things that were well below his capability, they would push him towards more challenging activities.
I think, with everything, it just depends. It depends on the kid, it depends on the school, the teachers, on everything, and the combination therein. We have a next door neighbor who has a daughter the same age as my son. They initially went to the same pre-k, and it was perfectly fine for her. My son was absolutely miserable.
Is this the first time your son has been away from you? If so, it may be separation anxiety combined with being compelled to do activities he is unaccustomed to.
If, however, he truly seems unhappy, I would urge you to pay attention and do what feels right. I shudder to think what my son would be like emotionally had I left him in that pre-k.
I wish it could be easier, I really do! I know how gut-wrenching it is to just "fly by the seat of your pants" and not know what is the "right" thing to do!
