I agree -- it depends so much on the teacher.

My DS was very unhappy his first year of casa (3-6 year old). The teacher was very rigid, had one style for everybody, and didn't allow "skipping steps". My DS wanted to do more advanced work, but she tried to force him to do all the preliminary steps (like learning to count to 10 and recognize the numbers when he was well beyond that). He spent way too much time sitting in the naughty chair.

We switched him to another teacher at the same school for the next two years of Casa and it was like night and day. I still have fond memories of his second teacher!

I suggest observing (they usually encourage that at Montessori) and talking to your son about why he is unhappy. My son at 4 years old told me he wanted to "break everything in the school" so that he wouldn't have to go back.