I went to Montessori as a child and don't remember having any issues with it (and had sort of vague fond memories of certain activities).
BUT we put our son in it. He was a late talker and is now showing some signs of dyslexia and it was a terrible place for him.
The way the teachers interpreted the curriculum was that every activity had to be done in a certain order. If you didn't "use the materials" (montessori children don't "play") in the prescribed order and in the prescribed way they were taken away from you. So for example when my son left his individual table and went to play imaginitively with what he considered to be "toys" with a friend at another table, he had materials taken away from him, and was eventually sent to time out. Same with when he went to go talk to his little friends during the morning "individual work." Nap time was of a ridiculous length (something like an hour and a half and he hadn't napped since eighteen mos) and he was not allowed to do anything but lie on his mat. Eventually he became so frustrated from all the time outs that he would spend naptime crying and they kicked him out of nursery for this. (I only found out when I asked- in fact they gave me written notice in his cubby which I opened in front of him. I had no warning whatsoever and was "informed" that he would be starting their regular program at another location in two weeks (in a class that was far too large, noisy, and disorganized).
I had to hold meetings with the Director of the whole group of centers to tell them that this was completely inappropriate and wound up putting him half-time in a smaller bilingual class until a full time spot opened up in that class later in the month.
I would second what others have said- go and observe for a whole day, both structured & unstructured time. Ask them about their philosophy re: using materials creatively & talking with others.
At this point I would only recommend it for a very few children- it really isn't appropriate for the average child and for kids who have learning disabilities it can be confounding and traumatizing.