OK, so posting from the perspective of a true Montessori school, I would say that it can work great for some gifted kids and be a disaster for others.
Overall, I think that the Montessori method works well, in general, for gifted kids when they are among the younger in their group. It is great when they are 6 or early 7 in the 6-9 class. The materials are set for them, up to 3 years ahead of their age level, they get to interact with older kids, etc. Then, when they get older (within the group), the material is less challenging or they might have finished it (some schools might be open to providing the child with the next level up materials, but some might not want it, or might not even have it); and then they are stuck with kids younger than them, who might be intellectually at a much lower level (due to asynchronous development).
Some PP mentioned the "chaos" of the Mont classroom. I think that, for the most part, Montessori preschools are much more quiet than play based and developmental preschools. Kids are supposed to be doing their work quietly. I think this is a plus for kids with some OE and too sensitive to noise, for example. This part was one that did NOT work for our rambuctious dd. She wanted to socialize more than anything else. Instead of choosing work that was appropriate and challenging for her (according to Maria Mont.'s theory), she'd choose works that she could do with a buddy or, as an alternative, works too hard for her to do on her own, so that the teacher would help. In short, socialization was driving her choices....
The other reason Mont. did NOT work for our dd (preschool), was that she is a very independent thinker, a divergent person. I take a look around in our family room right now and I see polly pockets sleeping in the Calico Critters house, and wooden furniture furnishing Lego creations. in Montessori, you can't combine materials from different things. You can't put the fuzzy balls with the cotton balls for pincer work. In the same sense, she always looks at novel ways of doing things. Montessori materials are self-correcting and are supposed to be used in a certain way.
We switched her to another preschool this year, more challenging in many ways, with a Reggio-Emilia base and constructivist base (among other things). The first thing we noticed in her is that now she talks about the exciting things she did or learned at school. Last year we only got reports of who sat with who, who held her hand, etc. We still get those, but now we get things like "wow! we are learning about germs, I am going to ask tomorrow if plants also get sick from germs!".
