DD9 has been in Montessori from the beginning and will be entering 5th grade in the Fall. We have had awesome teachers and a couple of bad ones--when they are good they are very very good, but when they've been bad they've been horrid. It has totally depended for us on the teacher rather than the school. The current teacher is great and we are just so happy with the situation. DD has been allowed to work at her own pace which generally works well except for math where the teacher doesn't feel she can really have a curriculum just for DD, but she is okay with us doing math outside as afterschooling and DD does mostly other subjects at school where she isn't so far ahead. So that's not perfect but it's our best option. This teacher and her classmates have been very patient and helpful in working with DD to develop her social skills (as she is Asperger-ish). My big concern is that this school only goes through 6th grade and the other schools around are all very different; hopefully DD will mature enough in the next year or so to be able to deal--I am optimistic. So if you are considering Montessori I would try to figure out how well the particular teacher is going to fit and sound them out about their attitudes towards kids working ahead--the great teachers have always been excited or at least curious at first about DD's abilities, and if you don't get that kind of response I would definitely have reservations and listen to your gut. Good luck!
