Momdeguerre (I LOVE your name, btw), dd1 has been going to a Montessori Children's House since she was 2.5 last september. the first thing to know is that there are many "degrees" of montessori.
She goes to a really pure montessori school which is rather extreme, but let me tell you my experiences.
It is at first a very dry environment like mummydoc says, in the sense that there are no obvious toys in a montessori school but rather "materials" (whcih are standard in all montessori institutions the world over) each designed to develop a particular skill. some of these materials look deceptively simple like the famous pink tower whcih is at first glance just stacking blocks until you realise the proportions and construction method are such as to give a child the intuition for the decimal system and square/cube roots etc.
there is a lot of emphasis on "life skills", ie dusting, cleaning, polishing, etc. to an adult watching it looks really boring, but children absolutely LOVE it and they are so proud of doing it right and they really enjoy it. there is a lot of emphasis on order and tidying up as a mark of respect for other children, and no structure like in other schools. children come in and choose their own activities under supervision of the teacher. some of it may be individual whereas others may be in pairs or groups.
my daughter's school is lovely, calm, very peaceful and the children are really well behaved in a very "energetic" way- they're all busy and all enjoying it and really learning through play, not running around aimlessly etc.
it is absolutely perfect for my dd and i really really love it, it has given her a lot of confidence and self-reliance, and the fact that there's older children is really lovely as there is an atmosphere of mutual support and team work.
hth!