Another Montessori family here, and we also LOVE it! My daughter has really grown and thrived in this environment. I think this methodology fosters independence and self-direction. And yes, my dd learned to read by 4, and now at 5 can read extremely well (e.g., she can read Harry Potter independently, which is written on approximately an 8-9 year old level). But I should add as I say this that, importantly, the point of Montessori is definitely NOT to "get ahead," and kids are not driven to achieve. It is a very self-paced style of learning.
The Montessori philosophy is "The environment itself will teach the child." The directresses set up a "prepared environment," with different materials. They give kids lessons on how to use different materials. Kids then have enormous freedom to do the work that they want to do at different times of day. Even snack is on your own schedule (or you can skip it altogether).
My dd is 5, and at her particular Montessori, once you turn 5, you are responsible for your "three things": one reading work, one math work, and one handwriting work every day. (She also has time to do other works.)
Here are some things some people might consider downsides to Montessori vs. traditional, and my thoughts:
1) Multi-age classrooms, e.g., 3-5s together, 6-8s together.
I worried about this, but it turns out to be GREAT. The 3-year-olds look up to the bigger kids and look forward to the day they will be able to do the things the bigger kids do. The older kids mentor and help the younger ones. Kids do work according to their individual ability levels, and are not marched lock-step with the other kids their age.
2) Student-teacher ratio.
I *really* worried about this before dd started. My dd's school allows up to a 1:12 teacher:student ratio. You will see much lower ratios at some traditional pre-schools, and of course as parents we want our kids to have plenty of attention...but the Montessori method allows it to work with a higher ratio. My dd's class has about 20 kids and 2 directresses at this point, but I think it has been as high as 24 kids with 2 directresses.
3) Some of the traditional play elements are sometimes not in Montessori classrooms, e.g., my daughter's classroom does not have a dress-up/imaginative play area. (It has Practical Living, but at least at my dd's school, it is not anywhere near the same.)
I do wish her Montessori had this, actually, but she does PLENTY of this after school, so I'm okay with it. (Not all Montessoris are the same, btw, so yours may have this. Some are more hard-core than others.)
By the way, our Montessori goes through MIDDLE SCHOOL. I have not yet decided if dd will go there all the way up to 8th grade, but we've loved her 3 years so far and we will definitely keep her there for next year (Lower Elementary).
Previous poster, I'm envious of the Chinese and piano lessons! At my dd's Montessori, they get Spanish a couple times a week, and she can do greetings, months, days of the week, body parts, etc. I do wish there was an option for more foreign language.