I LOVE the Montessori school my DD is in. She started out in a church preschool, but due to unannounced teacher changes, I could see their priorities and mine did not line up (they didn't think it was a big deal NOT to notify parents that the caregivers had changed

) so we switched immediately. Our local Montessori school had an opening so I toured/observed and fell in love with it.
Montessori is a very nurturning, academic program that encourages independence. THe programs are almost always multi-age classrooms, which allows the younger ones to learn from the older ones, and the older ones to practice their skills teaching the younger. For instance, my DD's class has 23 kids ages 2.5 and potty-trained through 5-year-old kindergardenters, with a lead teacher and 2 accredited assistants. The learning activities are all individualized. Each child works at his or her own pace, on activities of their choosing. This type of environment works very well for self-motivated children like my DD, but may not be for every kid.
It is a LOT more expensive than her other program was ($75/month for 2 mornings/week at the church preschool vs. $180/month for 2 mornings/week at the Montessori....AND she loved it so much we let her got a third morning for a total of $235/month!) but it's been worth every penny. Note that much of the expense is due to the educational level of the teachers (all of ours have degrees and Montessori certification....they don't hire the 18-year-old fresh out of HS with babysitting for work experience)....and the classrooms are very expensive to equip due to all the specialized equipment and activities. Our school added another preschool classroom this year and it cost $30,000+ to outfit it.
My DD LOVES her school. She learns so many things there, and not just the usual colors, shapes, numbers. They learn botany (parts of a flower, parts of a plant, etc), animal classification (mammal, vertebrates, etc.), art styles and themes, and she just started picking up the continents and states at the end of the year....oh....and that was her 2's year (she has a late birthday)! She'll do her 3-year-old year there, too, but then we move
After enduring the whole year with her mad at me on Mondays and Wednesdays that I won't take her to school, she'll be going 5 mornings in the fall.

for her,

for my checkbook. Most of the other Montessori schools in the area only offer 5 day programs.
I always pick out schools the year before so I have a chance to observe them in action. Is your school offering any summer activities that you could observe? This may help you make a decision.
Also, here's some reading on the Montessori philosophy...
http://www.montessori.org/ --look under FAQ's
http://www.montessori.edu/ --lots of good info here on the Montessori method, environments and materials, and a FAQ section
It is important to note that Montessori schools are all independently owned and operated, and there is no overarching standards or licensing body. As such, it's very important to check out your local school and how they implement the Montessori philosophy!
Good luck in your decision! I know I struggled with mine when I found the original school and felt just AWFUL when I realized I had to pull her out. It turned out to be a blessing in disguise, though, since that's how we found our wonderful school!