Steiner philosophy IMO is spooky and worrying. Basically Rudolf Steiner was an occultist Spiritualist. A lot of what kids actually do in the schools (eg the pictures they draw) relates to his rather odd beliefs. He also believed in the supremacy of white Germanic Europeans (he was writing about ooh 1920s I think, in Austria). Although the Steiner fellowship does a lot of good work with people with disabilities, one of his beliefs was that some disabiliteis were caused because a child rejected their parents just before birth, and also that some kids with disbilities were demons in disguise.
There is a religion/belief system/philopshy behind the schools. This is known as anthroposophy. From the POV of an anthroposophist (who in practice tend to comprise the board of trustees)the purpose of Steiner education is to allow the child to reincarnate sucessfully into their new body.
Steiner teachers do study his work. There tends to be a reverance for the teacher in a lot of schools and it can be impossible to criticise teaching methods because there is such a strong dogma behind them.
Another consideration is arts/craft and music. Both are highly stylised. A lot comes down to copying the teacher (this is deeply ingrained in the philosophy). Basically, creativity is often not much encouraged. Our ds so far loves drawing, dancing etc and I am concerned that a Steiner school would not allow him to do these things. Children are not allowed to listen to electronic media, including CDs/radio-this is in part because there is some evil demon of electricity which kids must be protected from (honestly, this is what they believe). They are also not allowed to draw in black crayon because black is an evil colour. See comments re white Germanic race above.
HOWEVER in practice Steiner schools offer kids things that they just don't get in the state system, eg prolonged time outdoors, lack of pressure to learn to read and write early, etc. Like anything else you can take good things from it. Just bear in mind that unlike Montessorri, Steiner schools are religious, and the exact nature of the religion is basically, to a lot of people, weird.
I spent time at a Steiner school and my mother is an anthroposophist. I also do take ds to a parent and toddler group at the local Steiner school,and have been to several lectuers.
Second Spacemonkey-have you thought of home-ed?